gitlab-ci: always run the containers build

If, the first time a fork was created, the job creating the containers was manually cancelled, this would have left the fork unable to use the CI (until the next automatic regeneration of the container, or a manual run of the CI).

Avoid this by always running the container-generation job, even though 99% of the time it will spin up, see that the container exists and shut down.

/cc @daenzer