Make Pull Requests run a build with tests

[vorburger]

@edewit it would be neat if we had CI on PRs for this project (again? can't remember if we used to).

We could EITHER set up something like Travis CI or Circle CI, or... maybe the Google Cloud Build which we're anyway working on for #259 could also do this? I'll try.

[vorburger]

https://github.com/OASIS-learn-study/minecraft-storeys-maker/runs/4046170262 shows how Google Cloud Build reports status... that's for building our develop branch - it's probably possible to somehow set this up for building each pull request as well.

[vorburger]

According to https://cloud.google.com/build/docs/automating-builds/build-repos-from-github (also https://cloud.google.com/build/docs/automating-builds/create-manage-triggers) this is trivial to set up - I've just done it!

@edewit NB that PRs from us will "auto-build", whereas from from our future army of contributors (LOL) 'collaborator or owner must comment "/gcbrun" to invoke trigger'.

Also worth noting is that this won't just ./gradlew, but just re-uses the Dockerfile - and apparently will actually push an image to a registry. I'm not sure how "clean-up" of such images works - I'll keep this issue open to figure that out. (BTW I've raised #265 to propose removing the -x test we currently have in there because of this.)

[edewit]

@vorburger we used to have a CI build I think it was Travis

[vorburger]

Pending #265, this is basically done now; I'll close this issue to clean up (as there is nothing additional to do here, other than that PR).