This also closes a slight bug we've had previously where the "postgres" user is _always_ created (now we only create the user specified via the environment variables).
Healthchecks that used `pg_isready -p 5432` were incorrectly flagging
the container as being healthy during initialization phase since
healthchecks are being run inside the container itself, so
`listen_addresses='localhost'` was not enough. Setting
`listen_addresses=''` forces the server to only listen on the unix
socket so no ports are open that might incorrectly interfeer with the
healthchecks.
