Call Docker in Container

Theory

There is no way to actually run a docker daemon in a container, but we can pass docker cli and docker daemon through volume mirroring.

Example

Let's run a container along with those we need mirrored.

docker run -it \
-v /usr/bin/docker:/usr/bin/docker \            ## mirror docker cli
-v /var/run/docker.sock:/var/run/docker.sock \  ## mirror docker.sock
ubuntu /bin/bash

Inside this container we can actually call the docker cli.

root@72f21d4335b99:/# docker info
Client:
 Context:    default
 Debug Mode: false
## other info ...

Use Case

The reason for me to do this is that I want to call docker cli in a gitlab-runner/docker executor during pipeline. The executor itself is a docker container.

Thus, I configured the runner this way:

[[runners]]
  [runners.docker]
    image = "ubuntu"
    volumes = [
        "/usr/bin/docker:/usr/bin/docker",
        "/var/run/docker.sock:/var/run/docker.sock",
        "/usr/local/bin/docker-compose:/usr/local/bin/docker-compose", 
        "/cache"
    ]