build-docker-image: Allow to run locally through a docker agent

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.

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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).

Ref. swh/infra/sysadm-environment#4724 (closed)