Using Docker Build

GitLab CI can 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 new trends in Continuous Integration/Deployment to:

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

It's also useful in case when your application already has the Dockerfile that can be used to create and test 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 build. This requires running GitLab Runner in privileged mode which can be harmful when untrusted code is run.

There are two methods to enable the use of docker build and docker run during build.

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 http://ci.gitlab.com \
     --token RUNNER_TOKEN \
     --executor shell
     --description "My Runner"
   

3. Install Docker on server.

   For more information how to install Docker 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

Second approach is to use special Docker image with all tools installed (docker and docker-compose) and run 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 command line to use docker and privileged mode:

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

   The above command will register new Runner to use special gitlab/dind image which is provided by GitLab Inc. The image at the start runs Docker daemon in docker-in-docker mode.

3. You can now use docker from build script:

   before_script:
     - docker info
   
   build_image:
     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 disables all security mechanisms of containers and exposing your host to privilege escalation which can lead to container breakout. For more information you could be interested in checking out Runtime privilege.