dev/sg: fix redis-postgres persistence

Created by: bobheadxi

Looks like the new postgres-12.6 image has a custom PGDATA set (https://github.com/sourcegraph/sourcegraph/pull/22414) - which means that no volume has been attached to Postgres, so anyone using redis-postgres would not have had any database persistence. This updates the volume mount to point to the correct directory.

This change also persists pg data to disk by default because that's what all the other dev services seem to be doing too ($HOME/.sourcegraph-dev/data/...)