Put docker image on GHCR

Currently there is an image available for `pgstac` on GHCR, i.e. `ghcr.io/stac-utils/pgstac`, but there does not appear to be an image for `stac-fastapi`, which would be helpful.

Do you all of plans of doing that?

I've created a lightweight pytest fixture set that runs stac-fastapi, so that I can test `pystac-client` calls against it, and this would simplify things.