remove docker compose

See merge request !6

Instead of hardcoding 2 runners to be registered, you can add profiles
to a directory to specify what runners should be registered.

This is moved to a separate repository so that the image can be used
for more usecases.

Instead of modifying the docker-compose file, it's easier and better to
use a `.env` file where you can provide the parameters for this runner.

This is handled by docker-compose

Nothing is ever stored in the home dir, so don't bother to make a mount
for it.

See merge request !5

In order to be able to run docker images inside docker, we mount the docker
socket inside the build container. This offer a security risk, as any CI job is
able interact with the docker engine the host.

To mitigate this, one runner is used to build docker images. This runner
should be limited to trusted projects.

The other runner can be shared, as it does not get the docker socket.

See merge request !4

Dockerfile: use gitlab-runner from community

See merge request !3

gitlab-runner has been moved to community, so we need to obtain it from
there. As we were getting gitlab-runner from edge anyway, switch the
image to use edge.

The gitlab-runner alpine package now creates the user, so we no longer
need to do this in the Dockerfile.

Improve build code

See merge request !2

Instead of having to manually run a command to register the runner,
automatically register the runner on start if the config file does not
exist yet.

In order for jobs on this runner to be able to build docker images, the
build container needs access to docker.

One option is to use docker-in-docker, but this is not a recommended
option.

The other option is to mount the docker socket from the host to the
build containers. This is technically the best option, but it comes with
some issues:

* CI jobs get full access to docker, with all it's security implications
* Different jobs can create conflicting images / containers
* Jobs could create images which remain on the host, and could be
  accidentaly (or on purpose) be used by other jobs. This is limited by
  the setting to always fetch images from the registry.

This remains a trade-off between security and convenience, but it should
be used with care.

add docker file to build the image

See merge request alpine/infra/gitlab-runner!1