CI Fixes - don't auto-build weekly, don't run duplicate jobs

[AngusP]

• I'm not sure we should rebuild the docker image on a schedule, it can break things unexpectedly without code or package changes, e.g. getting python error #13 and makes it seem like there's been an update when there hasn't.

• Separatelty, some of the triggers for CI runs seemingly overlap so we run the same job twice/whatever (e.g. on Package Updates, downgrade to Py3.11 for Pydantic compatibility issue. Fixes #13 #14)

• Also, we just check that the image starts up ok in CI. We're not running the same tests that just test does -- a separate test job would be good.