Prepare GMT for use inside / outside Gitlab runners

[ArneTR]

We recently discussed the option to encapsulate a Gitlab runner in the GMT or to have a Gitlab runner encapsulating the GMT.

In both cases the first functionality we would need is to make one-off runs and not have the runner code constantly in a loop waiting for new jobs to process.

@dan-mm Could you make a first investigation after your vacation, so that @ribalba can pick this up after his vacation.