Move the GitLab runner from simba.i.o.o to mickey.i.o.o
This is a cherry-picked / updated commit, based on !215 originally
proposed by @tampakrap. It was on hold for a long time, and I've updated
it, so it cleanly applies to current `production`.
Once this is deployed, we can finally get rid of `simba.i.o.o` to safe
on ressources.