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 new trends in Continuous Integration/Deployment to:

create application image,
run test against created image,
push image to remote registry,
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.

Install GitLab Runner.

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"

Install Docker Engine on server.

For more information how to install Docker Engine on different systems checkout the Supported installations.
Add gitlab-runner user to docker group:
```
$ sudo usermod -aG docker gitlab-runner
```

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

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

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/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.

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

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.