Building Docker images with GitLab CI/CD

You can use GitLab CI/CD with Docker Engine to build and test Docker-based projects.

For example, you might want to:

Create an application image.
Run tests against the created image.
Push image to a remote registry.
Deploy to a server from the pushed image.

Or, if your application already has a Dockerfile, you can use it to create and test an image:

docker build -t my-image dockerfiles/
docker run my-image /script/to/run/tests
docker tag my-image my-registry:5000/my-image
docker push my-registry:5000/my-image

To run Docker commands in your CI/CD jobs, you must configure GitLab Runner to enable docker support.

Enable Docker commands in your CI/CD jobs

There are three ways to enable the use of docker build and docker run during jobs, each with their own tradeoffs. You can use:

The shell executor
The Docker executor with the Docker image (Docker-in-Docker)
Docker socket binding

If you don't want to execute a runner in privileged mode, but want to use docker build, you can also use kaniko.

If you are using shared runners on GitLab.com, see GitLab.com shared runners to learn more about how these runners are configured.

Use the shell executor

One way to configure GitLab Runner for docker support is to use the shell executor.

After you register a runner and select the shell executor, your job scripts are executed as the gitlab-runner user. This user needs permission to run Docker commands.

Install GitLab Runner.

sudo gitlab-runner register -n \
  --url https://gitlab.com/ \
  --registration-token REGISTRATION_TOKEN \
  --executor shell \
  --description "My Runner"

On the server where GitLab Runner is installed, install Docker Engine. View a list of supported platforms.
Add the gitlab-runner user to the docker group:
```
sudo usermod -aG docker gitlab-runner
```
Verify that gitlab-runner has access to Docker:
```
sudo -u gitlab-runner -H docker info
```

In GitLab, to verify that everything works, add docker info to .gitlab-ci.yml:

before_script:
  - docker info

build_image:
  script:
    - docker build -t my-docker-image .
    - docker run my-docker-image /script/to/run/tests

You can now use docker commands (and install docker-compose if needed).

When you add gitlab-runner to the docker group, you are effectively granting gitlab-runner full root permissions. Learn more about the security of the docker group.

Use the Docker executor with the Docker image (Docker-in-Docker)

Another way to configure GitLab Runner for docker support is to register a runner with the Docker executor and use the Docker image to run your job scripts. This configuration is referred to as "Docker-in-Docker."

The Docker image has all of the docker tools installed and can run the job script in context of the image in privileged mode.

The docker-compose command is not available in this configuration by default. To use docker-compose in your job scripts, follow the docker-compose installation instructions.

WARNING: When you enable --docker-privileged, you are effectively disabling all of the security mechanisms of containers and exposing your host to privilege escalation which can lead to container breakout. For more information, check out the official Docker documentation on runtime privilege and Linux capabilities.

Docker-in-Docker works well, and is the recommended configuration, but it is not without its own challenges:

When using Docker-in-Docker, each job is in a clean environment without the past history. Concurrent jobs work fine because every build gets its own instance of Docker engine so they don't conflict with each other. But this also means that jobs can be slower because there's no caching of layers.
By default, Docker 17.09 and higher uses --storage-driver overlay2 which is the recommended storage driver. See Using the overlayfs driver for details.
Since the docker:19.03.12-dind container and the runner container don't share their root file system, the job's working directory can be used as a mount point for child containers. For example, if you have files you want to share with a child container, you may create a subdirectory under /builds/$CI_PROJECT_PATH and use it as your mount point (for a more thorough explanation, check issue #41227):
```
variables:
  MOUNT_POINT: /builds/$CI_PROJECT_PATH/mnt

script:
  - mkdir -p "$MOUNT_POINT"
  - docker run -v "$MOUNT_POINT:/mnt" my-docker-image
```

An example project using this approach can be found here: https://gitlab.com/gitlab-examples/docker.

In the examples below, we are using Docker images tags to specify a specific version, such as docker:19.03.12. If tags like docker:stable are used, you have no control over what version is used. This can lead to unpredictable behavior, especially when new versions are released.

TLS enabled

The Docker daemon supports connection over TLS and it's done by default for Docker 19.03.12 or higher. This is the suggested way to use the Docker-in-Docker service and GitLab.com shared runners support this.

Docker

Introduced in GitLab Runner 11.11.

Install GitLab Runner.
Register GitLab Runner from the command line to use docker and privileged mode:
```
sudo gitlab-runner register -n \
  --url https://gitlab.com/ \
  --registration-token REGISTRATION_TOKEN \
  --executor docker \
  --description "My Docker Runner" \
  --docker-image "docker:19.03.12" \
  --docker-privileged \
  --docker-volumes "/certs/client"
```
The above command registers a new runner to use the special docker:19.03.12 image, which is provided by Docker. Notice that it's using the privileged mode to start the build and service containers. If you want to use Docker-in-Docker mode, you always have to use privileged = true in your Docker containers.

This also mounts /certs/client for the service and build container, which is needed for the Docker client to use the certificates inside of that directory. For more information on how Docker with TLS works, check https://hub.docker.com/_/docker/#tls.

The above command creates a config.toml entry similar to this:
```
[[runners]]
  url = "https://gitlab.com/"
  token = TOKEN
  executor = "docker"
  [runners.docker]
    tls_verify = false
    image = "docker:19.03.12"
    privileged = true
    disable_cache = false
    volumes = ["/certs/client", "/cache"]
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]
```

You can now use docker in the build script (note the inclusion of the docker:19.03.12-dind service):

image: docker:19.03.12

variables:
  # When using dind service, we need to instruct docker to talk with
  # the daemon started inside of the service. The daemon is available
  # with a network connection instead of the default
  # /var/run/docker.sock socket. Docker 19.03 does this automatically
  # by setting the DOCKER_HOST in
  # https://github.com/docker-library/docker/blob/d45051476babc297257df490d22cbd806f1b11e4/19.03/docker-entrypoint.sh#L23-L29
  #
  # The 'docker' hostname is the alias of the service container as described at
  # https://docs.gitlab.com/ee/ci/docker/using_docker_images.html#accessing-the-services.
  #
  # Specify to Docker where to create the certificates, Docker will
  # create them automatically on boot, and will create
  # `/certs/client` that will be shared between the service and job
  # container, thanks to volume mount from config.toml
  DOCKER_TLS_CERTDIR: "/certs"

services:
  - docker:19.03.12-dind

before_script:
  - docker info

build:
  stage: build
  script:
    - docker build -t my-docker-image .
    - docker run my-docker-image /script/to/run/tests

Kubernetes

Introduced in GitLab Runner Helm Chart 0.23.0.

Using the Helm chart, update the values.yml file to specify a volume mount.

runners:
  config: |
    [[runners]]
      [runners.kubernetes]
        image = "ubuntu:20.04"
        privileged = true
      [[runners.kubernetes.volumes.empty_dir]]
        name = "docker-certs"
        mount_path = "/certs/client"
        medium = "Memory"

You can now use docker in the build script (note the inclusion of the docker:19.03.13-dind service):

image: docker:19.03.13

variables:
  # When using dind service, we need to instruct docker to talk with
  # the daemon started inside of the service. The daemon is available
  # with a network connection instead of the default
  # /var/run/docker.sock socket.
  DOCKER_HOST: tcp://docker:2376
  #
  # The 'docker' hostname is the alias of the service container as described at
  # https://docs.gitlab.com/ee/ci/docker/using_docker_images.html#accessing-the-services.
  # If you're using GitLab Runner 12.7 or earlier with the Kubernetes executor and Kubernetes 1.6 or earlier,
  # the variable must be set to tcp://localhost:2376 because of how the
  # Kubernetes executor connects services to the job container
  # DOCKER_HOST: tcp://localhost:2376
  #
  # Specify to Docker where to create the certificates, Docker will
  # create them automatically on boot, and will create
  # `/certs/client` that will be shared between the service and job
  # container, thanks to volume mount from config.toml
  DOCKER_TLS_CERTDIR: "/certs"
  # These are usually specified by the entrypoint, however the
  # Kubernetes executor doesn't run entrypoints
  # https://gitlab.com/gitlab-org/gitlab-runner/-/issues/4125
  DOCKER_TLS_VERIFY: 1
  DOCKER_CERT_PATH: "$DOCKER_TLS_CERTDIR/client"

services:
  - docker:19.03.13-dind

before_script:
  - docker info

build:
  stage: build
  script:
    - docker build -t my-docker-image .
    - docker run my-docker-image /script/to/run/tests

TLS disabled

Sometimes there are legitimate reasons why you might want to disable TLS. For example, you have no control over the GitLab Runner configuration that you are using.

Assuming that the runner's config.toml is similar to:

[[runners]]
  url = "https://gitlab.com/"
  token = TOKEN
  executor = "docker"
  [runners.docker]
    tls_verify = false
    image = "docker:19.03.12"
    privileged = true
    disable_cache = false
    volumes = ["/cache"]
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]

You can now use docker in the build script (note the inclusion of the docker:19.03.12-dind service):

image: docker:19.03.12

variables:
  # When using dind service we need to instruct docker, to talk with the
  # daemon started inside of the service. The daemon is available with
  # a network connection instead of the default /var/run/docker.sock socket.
  #
  # The 'docker' hostname is the alias of the service container as described at
  # https://docs.gitlab.com/ee/ci/docker/using_docker_images.html#accessing-the-services
  #
  # If you're using GitLab Runner 12.7 or earlier with the Kubernetes executor and Kubernetes 1.6 or earlier,
  # the variable must be set to tcp://localhost:2375 because of how the
  # Kubernetes executor connects services to the job container
  # DOCKER_HOST: tcp://localhost:2375
  #
  DOCKER_HOST: tcp://docker:2375
  #
  # This will instruct Docker not to start over TLS.
  DOCKER_TLS_CERTDIR: ""

services:
  - docker:19.03.12-dind

before_script:
  - docker info

build:
  stage: build
  script:
    - docker build -t my-docker-image .
    - docker run my-docker-image /script/to/run/tests

Use Docker socket binding

Another way to configure GitLab Runner for docker support is to bind-mount /var/run/docker.sock into the container so that Docker is available in the context of the image.

NOTE: If you bind the Docker socket and you are using GitLab Runner 11.11 or later, you can no longer use docker:19.03.12-dind as a service. Volume bindings are done to the services as well, making these incompatible.

To make Docker available in the context of the image:

Install GitLab Runner.

From the command line, register a runner with the docker executor and share /var/run/docker.sock:

sudo gitlab-runner register -n \
  --url https://gitlab.com/ \
  --registration-token REGISTRATION_TOKEN \
  --executor docker \
  --description "My Docker Runner" \
  --docker-image "docker:19.03.12" \
  --docker-volumes /var/run/docker.sock:/var/run/docker.sock

This command registers a new runner to use the special docker:19.03.12 image, which is provided by Docker. The command uses the Docker daemon of the runner itself. Any containers spawned by Docker commands are siblings of the runner rather than children of the runner. This may have complications and limitations that are unsuitable for your workflow.

Your config.toml file should now have an entry like this:

[[runners]]
  url = "https://gitlab.com/"
  token = REGISTRATION_TOKEN
  executor = "docker"
  [runners.docker]
    tls_verify = false
    image = "docker:19.03.12"
    privileged = false
    disable_cache = false
    volumes = ["/var/run/docker.sock:/var/run/docker.sock", "/cache"]
  [runners.cache]
    Insecure = false

Use docker in the build script. You don't need to include the docker:19.03.12-dind service, like you do when you're using the Docker-in-Docker executor:

image: docker:19.03.12

before_script:
  - docker info

build:
  stage: build
  script:
    - docker build -t my-docker-image .
    - docker run my-docker-image /script/to/run/tests

This method avoids using Docker in privileged mode. However, the implications of this method are:

By sharing the Docker daemon, you are effectively disabling all the security mechanisms of containers and exposing your host to privilege escalation, which can lead to container breakout. For example, if a project ran docker rm -f $(docker ps -a -q) it would remove the GitLab Runner containers.
Concurrent jobs may not work; if your tests create containers with specific names, they may conflict with each other.
Sharing files and directories from the source repository into containers may not work as expected. Volume mounting is done in the context of the host machine, not the build container. For example:
```
docker run --rm -t -i -v $(pwd)/src:/home/app/src test-image:latest run_app_tests
```

Enable registry mirror for `docker:dind` service

When the Docker daemon starts inside of the service container, it uses the default configuration. You may want to configure a registry mirror for performance improvements and ensuring you don't reach DockerHub rate limits.

Inside `.gitlab-ci.yml`

You can append extra CLI flags to the dind service to set the registry mirror:

services:
   - name: docker:19.03.13-dind
     command: ["--registry-mirror", "https://registry-mirror.example.com"] # Specify the registry mirror to use.

DinD service defined inside of GitLab Runner configuration

Introduced in GitLab Runner 13.6.

If you are an administrator of GitLab Runner and you have the dind service defined for the Docker executor, or the Kubernetes executor you can specify the command to configure the registry mirror for the Docker daemon.

Docker:

[[runners]]
  ...
  executor = "docker"
  [runners.docker]
    ...
    privileged = true
    [[runners.docker.services]]
      name = "docker:19.03.13-dind"
      command = ["--registry-mirror", "https://registry-mirror.example.com"]

Kubernetes:

[[runners]]
  ...
  name = "kubernetes"
  [runners.kubernetes]
    ...
    privileged = true
    [[runners.kubernetes.services]]
      name = "docker:19.03.13-dind"
      command = ["--registry-mirror", "https://registry-mirror.example.com"]

Docker executor inside GitLab Runner configuration

If you are an administrator of GitLab Runner and you want to use the mirror for every dind service, update the configuration to specify a volume mount.

For example, if you have a /opt/docker/daemon.json file with the following content:

{
  "registry-mirrors": [
    "https://registry-mirror.example.com"
  ]
}

Update the config.toml file to mount the file to /etc/docker/daemon.json. This would mount the file for every container that is created by GitLab Runner. The configuration is picked up by the dind service.

[[runners]]
  ...
  executor = "docker"
  [runners.docker]
    image = "alpine:3.12"
    privileged = true
    volumes = ["/opt/docker/daemon.json:/etc/docker/daemon.json:ro"]

Kubernetes executor inside GitLab Runner configuration

Introduced in GitLab Runner 13.6.

If you are an administrator of GitLab Runner and you want to use the mirror for every dind service, update the configuration to specify a ConfigMap volume mount.

For example, if you have a /tmp/daemon.json file with the following content:

{
  "registry-mirrors": [
    "https://registry-mirror.example.com"
  ]
}

Create a ConfigMap with the content of this file. You can do this with a command like:

kubectl create configmap docker-daemon --namespace gitlab-runner --from-file /tmp/daemon.json

NOTE: Make sure to use the namespace that GitLab Runner Kubernetes executor uses to create job pods in.

After the ConfigMap is created, you can update the config.toml file to mount the file to /etc/docker/daemon.json. This update mounts the file for every container that is created by GitLab Runner. The configuration is picked up by the dind service.

[[runners]]
  ...
  executor = "kubernetes"
  [runners.kubernetes]
    image = "alpine:3.12"
    privileged = true
    [[runners.kubernetes.volumes.config_map]]
      name = "docker-daemon"
      mount_path = "/etc/docker/daemon.json"
      sub_path = "daemon.json"

Authenticating with registry in Docker-in-Docker

When you use Docker-in-Docker, the normal authentication methods won't work because a fresh Docker daemon is started with the service.

Option 1: Run `docker login`

In before_script run docker login:

image: docker:19.03.13

variables:
  DOCKER_TLS_CERTDIR: "/certs"

services:
  - docker:19.03.13-dind

build:
  stage: build
  before_script:
    - echo "$DOCKER_REGISTRY_PASS" | docker login $DOCKER_REGISTRY --username $DOCKER_REGISTRY_USER --password-stdin
  script:
    - docker build -t my-docker-image .
    - docker run my-docker-image /script/to/run/tests

To log in to Docker Hub, leave $DOCKER_REGISTRY empty or remove it.

Option 2: Mount `~/.docker/config.json` on each job

If you are an administrator for GitLab Runner, you can mount a file with the authentication configuration to ~/.docker/config.json. Then every job that the runner picks up will be authenticated already. If you are using the official docker:19.03.13 image, the home directory is under /root.

If you mount the configuration file, any docker command that modifies the ~/.docker/config.json (for example, docker login) fails, because the file is mounted as read-only. Do not change it from read-only, because other problems will occur.

Here is an example of /opt/.docker/config.json that follows the DOCKER_AUTH_CONFIG documentation:

{
    "auths": {
        "https://index.docker.io/v1/": {
            "auth": "bXlfdXNlcm5hbWU6bXlfcGFzc3dvcmQ="
        }
    }
}

Docker

Update the volume mounts to include the file.

[[runners]]
  ...
  executor = "docker"
  [runners.docker]
    ...
    privileged = true
    volumes = ["/opt/.docker/config.json:/root/.docker/config.json:ro"]

Kubernetes

Create a ConfigMap with the content of this file. You can do this with a command like:

kubectl create configmap docker-client-config --namespace gitlab-runner --from-file /opt/.docker/config.json

Update the volume mounts to include the file.

[[runners]]
  ...
  executor = "kubernetes"
  [runners.kubernetes]
    image = "alpine:3.12"
    privileged = true
    [[runners.kubernetes.volumes.config_map]]
      name = "docker-client-config"
      mount_path = "/root/.docker/config.json"
      # If you are running GitLab Runner 13.5
      # or lower you can remove this
      sub_path = "config.json"

Option 3: Use `DOCKER_AUTH_CONFIG`

If you already have DOCKER_AUTH_CONFIG defined, you can use the variable and save it in ~/.docker/config.json.

There are multiple ways to define this. For example:

Inside pre_build_script inside of the runner configuration file.
Inside before_script.
Inside of script.

Below is an example of before_script. The same commands apply for any solution you implement.

image: docker:19.03.13

variables:
  DOCKER_TLS_CERTDIR: "/certs"

services:
  - docker:19.03.13-dind

build:
  stage: build
  before_script:
    - mkdir -p $HOME/.docker
    - echo $DOCKER_AUTH_CONFIG > $HOME/.docker/config.json
  script:
    - docker build -t my-docker-image .
    - docker run my-docker-image /script/to/run/tests

Making Docker-in-Docker builds faster with Docker layer caching

When using Docker-in-Docker, Docker downloads all layers of your image every time you create a build. Recent versions of Docker (Docker 1.13 and above) can use a pre-existing image as a cache during the docker build step, considerably speeding up the build process.

How Docker caching works

When running docker build, each command in Dockerfile results in a layer. These layers are kept around as a cache and can be reused if there haven't been any changes. Change in one layer causes all subsequent layers to be recreated.

You can specify a tagged image to be used as a cache source for the docker build command by using the --cache-from argument. Multiple images can be specified as a cache source by using multiple --cache-from arguments. Keep in mind that any image that's used with the --cache-from argument must first be pulled (using docker pull) before it can be used as a cache source.

Using Docker caching

Here's a .gitlab-ci.yml file showing how Docker caching can be used:

image: docker:19.03.12

services:
  - docker:19.03.12-dind

variables:
  # Use TLS https://docs.gitlab.com/ee/ci/docker/using_docker_build.html#tls-enabled
  DOCKER_HOST: tcp://docker:2376
  DOCKER_TLS_CERTDIR: "/certs"

before_script:
  - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY

build:
  stage: build
  script:
    - docker pull $CI_REGISTRY_IMAGE:latest || true
    - docker build --cache-from $CI_REGISTRY_IMAGE:latest --tag $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA --tag $CI_REGISTRY_IMAGE:latest .
    - docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA
    - docker push $CI_REGISTRY_IMAGE:latest

The steps in the script section for the build stage can be summed up to:

The first command tries to pull the image from the registry so that it can be used as a cache for the docker build command.
The second command builds a Docker image using the pulled image as a cache (notice the --cache-from $CI_REGISTRY_IMAGE:latest argument) if available, and tags it.
The last two commands push the tagged Docker images to the container registry so that they may also be used as cache for subsequent builds.

Use the OverlayFS driver

NOTE: The shared runners on GitLab.com use the overlay2 driver by default.

By default, when using docker:dind, Docker uses the vfs storage driver which copies the filesystem on every run. This is a disk-intensive operation which can be avoided if a different driver is used, for example overlay2.

Requirements

Make sure a recent kernel is used, preferably >= 4.2.
Check whether the overlay module is loaded:
```
sudo lsmod | grep overlay
```
If you see no result, then it isn't loaded. To load it use:
```
sudo modprobe overlay
```
If everything went fine, you need to make sure module is loaded on reboot. On Ubuntu systems, this is done by editing /etc/modules. Just add the following line into it:
```
overlay
```

Use the OverlayFS driver per project

You can enable the driver for each project individually by using the DOCKER_DRIVER environment variable in .gitlab-ci.yml:

variables:
  DOCKER_DRIVER: overlay2

Use the OverlayFS driver for every project

If you use your own GitLab Runners, you can enable the driver for every project by setting the DOCKER_DRIVER environment variable in the [[runners]] section of config.toml:

environment = ["DOCKER_DRIVER=overlay2"]

If you're running multiple runners, you have to modify all configuration files.

Read more about the runner configuration and using the OverlayFS storage driver.

Using the GitLab Container Registry

After you've built a Docker image, you can push it up to the built-in GitLab Container Registry.

Troubleshooting

`docker: Cannot connect to the Docker daemon at tcp://docker:2375. Is the docker daemon running?`

This is a common error when you are using Docker in Docker v19.03 or higher.

This occurs because Docker starts on TLS automatically, so you need to do some setup. If:

This is the first time setting it up, carefully read using Docker in Docker workflow.
You are upgrading from v18.09 or earlier, read our upgrade guide.

Building Docker images with GitLab CI/CD

Enable Docker commands in your CI/CD jobs

Use the shell executor

Use the Docker executor with the Docker image (Docker-in-Docker)

TLS enabled

Docker

Kubernetes

TLS disabled

Use Docker socket binding

Enable registry mirror for docker:dind service

Inside .gitlab-ci.yml

DinD service defined inside of GitLab Runner configuration

Docker executor inside GitLab Runner configuration

Kubernetes executor inside GitLab Runner configuration

Authenticating with registry in Docker-in-Docker

Option 1: Run docker login

Option 2: Mount ~/.docker/config.json on each job

Docker

Kubernetes

Option 3: Use DOCKER_AUTH_CONFIG

Making Docker-in-Docker builds faster with Docker layer caching

How Docker caching works

Using Docker caching

Use the OverlayFS driver

Requirements

Use the OverlayFS driver per project

Use the OverlayFS driver for every project

Using the GitLab Container Registry

Troubleshooting

docker: Cannot connect to the Docker daemon at tcp://docker:2375. Is the docker daemon running?

Enable registry mirror for `docker:dind` service

Inside `.gitlab-ci.yml`

Option 1: Run `docker login`

Option 2: Mount `~/.docker/config.json` on each job

Option 3: Use `DOCKER_AUTH_CONFIG`

`docker: Cannot connect to the Docker daemon at tcp://docker:2375. Is the docker daemon running?`