This switches us to a container image maintained by the
matrix-registration developer.

0.7.2 also supports a `base_url` configuration option we can use to
make it easier to reverse-proxy at a different base URL.

We still keep some workarounds, because of this issue:
https://github.com/ZerataX/matrix-registration/issues/47

Auto-migration and everything seems to work. It's just that
matrix-registration cannot load the Python modules required
for talking to a Postgres database.

Tracked here: https://github.com/ZerataX/matrix-registration/issues/44

Until this gets fixed, we'll continue default to 'sqlite'.