Some job using docker cannot easily be built using the host since they require some mount points to happen.
Using a docker agent ease such build.
So this mr proposes to update the instruction to be able to reproduce some of those builds:
build-docker-image
(ongoing) build-docker-images
This focuses mainly on the job build-docker-image to ensure this actually works.
Other jobs should mostly work. This is still a work in progress as we would need
a way to declaratively deactivate some chronic jobs but baby steps.
This introduces a new variable node which allows to either run it on the built-in
node (like actually with the production jenkins) or on the 'docker' agent node (which
should be configured either prior to choose it).
This is actually useful for running in a local jenkins and hacking around those jobs.
This will also allow further developement on similar jobs which requires to run docker.
For example, the future build-docker-images.
This also drops the instruction updateGitlabCommitStatus which is not needed in that
job (since that won't be opened in a mr context).
