gitlab-org--gitlab-foss/doc/ci/docker/using_docker_build.md

Using Docker Build

GitLab CI allows you to use Docker Engine to build and test docker-based projects.

This also allows to you to use docker-compose and other docker-enabled tools.

This is one of the new trends in Continuous Integration/Deployment to:

1. create an application image,
2. run tests against the created image,
3. push image to a remote registry,
4. deploy server from the pushed image

It's also useful when your application already has the Dockerfile that can be used to create and test an image:

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

However, this requires special configuration of GitLab Runner to enable docker support during builds. There are three methods to enable the use of docker build and docker run during builds.

1. Use shell executor

The simplest approach is to install GitLab Runner in shell execution mode. GitLab Runner then executes build scripts as gitlab-runner user.

1. Install GitLab Runner.

2. During GitLab Runner installation select shell as method of executing build scripts or use command:

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

3. Install Docker Engine on server.

   For more information how to install Docker Engine on different systems checkout the Supported installations.

4. Add gitlab-runner user to docker group:

   $ sudo usermod -aG docker gitlab-runner
   

5. Verify that gitlab-runner has access to Docker:

   $ sudo -u gitlab-runner -H docker info
   

   You can now verify that everything works by adding 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
   

6. You can now use docker command and install docker-compose if needed.

7. However, by adding gitlab-runner to docker group you are effectively granting gitlab-runner full root permissions. For more information please checkout On Docker security: docker group considered harmful.

2. Use docker-in-docker executor

The second approach is to use the special Docker image with all tools installed (docker and docker-compose) and run the build script in context of that image in privileged mode.

In order to do that follow the steps:

1. Install GitLab Runner.

2. Register GitLab Runner from the command line to use docker and privileged mode:

   sudo gitlab-runner register -n \
     --url https://gitlab.com/ci \
     --token RUNNER_TOKEN \
     --executor docker \
     --description "My Docker Runner" \
     --docker-image "docker:latest" \
     --docker-privileged
   

   The above command will register a new Runner to use the special docker:latest 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.

   The above command will create a config.toml entry similar to this:

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

   If you want to use the Shared Runners available on your GitLab CE/EE installation in order to build Docker images, then make sure that your Shared Runners configuration has the privileged mode set to true.

3. You can now use docker from build script:

   image: docker:latest
   
   services:
   - docker:dind
   
   before_script:
   - docker info
   
   build:
     stage: build
     script:
     - docker build -t my-docker-image .
     - docker run my-docker-image /script/to/run/tests
   

4. However, by enabling --docker-privileged you are effectively disabling all 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.

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

3. Bind Docker socket

The third approach is to bind-mount /var/run/docker.sock into the container so that docker is available in the context of that image.

In order to do that follow the steps:

1. Install GitLab Runner.

2. Register GitLab Runner from the command line to use docker and privileged mode:

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

   The above command will register a new Runner to use the special docker:latest image which is provided by Docker. Notice that it's using the Docker daemon of the runner itself, and any containers spawned by docker commands will be siblings of the runner rather than children of the runner. This may have complications and limitations that are unsuitable for your workflow.

   The above command will create a config.toml entry similar to this:

   [[runners]]
     url = "https://gitlab.com/ci"
     token = TOKEN
     executor = "docker"
     [runners.docker]
       tls_verify = false
       image = "docker:latest"
       privileged = false
       disable_cache = false
       volumes = ["/usr/local/bin/docker:/usr/bin/docker", "/cache"]
     [runners.cache]
       Insecure = false
   

3. You can now use docker from build script (note that you don't need to include the docker:dind service as in the option above):

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

4. However, 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.