You can still connect the DB with a PgBouncer image spun up in another container however. Unfortunately, I can't really recommend you which one since there doesn't seem to be an official Docker image for PgBouncer and I myself have never tried any of the existing ones out there. If you're looking to use PostgREST however, they do have an official Docker image that you can use over here [4].
[0]: https://hub.docker.com/r/supabase/postgres
[1]: https://github.com/supabase/postgres#extensions
[2]: https://github.com/supabase/postgres#enhanced-security
[3]: https://github.com/supabase/postgres#additional-goodies
But recently I got the occasion to work with https://github.com/CrunchyData/postgres-operator .
It's a bit rough around the edges wit the v5, which is very recent, but overall I was really happy with it.
I am still not using it in production but it's on the way.
One other request - could you write how to use it (connect to it) ?