This adds Gitlab CI to Alexandria using my (still WIP) Gitlab CI helpers that I announced at https://mailman.common-lisp.net/pipermail/clo-devel/2021-November/001508.html
These are still not quite complete, but I'm pretty confident that the features I've enabled in this MR will not change (and if they do, I'll reach out again).
I've enabled Allegro testing because my understanding is that using it for this complies with the Allegro Express license. Let me know if you're happy with that decision as well.
The main feature not currently enabled is the "release" feature. It integrates with Gitlab to mark pushed tags as releases. We can enable it if you want, but as currently written it requires a Changelog to be kept.