Create .gitlab-ci.yml

[umlaeute]

This is partB of a revamped version of #504, that only includes the ".gitlab-ci.yml" changes.

this PR has squashed the various commits that change this file.

please note that there are some things i find a bit weird about this PR:

• this uses syso/giada-builder as the Docker base image, rather than monocasual/giada-docker
• I don't know anything about the details, but the latter seems to be the more natural choice...
• it seems that giada is not using GitLab at all, so I see little point in having a CI-configuration for it.
  • I have rather strong feelings about git-providers encouraging people to leave their hoster-specific marks everywhere in the repository
  • GitLab at least offers the possibility, to add a URL as the CI-configuration, and you can do that in the project settings. so you don't really need to put that file into the repository itself (esp. if you don't use GitLab as the main provider)

[gvnnz]

Yeah, I actually don't know why this PR was proposed in the first place. We run our CI on GitHub Actions, adding another one would be a) pointless b) a maintenance burden. Thanks anyway for the effort!