postgres:18 (tracked by pgautoupgrade) relocated the default PGDATA from
/var/lib/postgresql/data to /var/lib/postgresql/18/docker. Since the db volume
is mounted at the legacy path, pg18 would init a fresh empty cluster there and
orphan all existing data (pgautoupgrade never runs the upgrade). Pinning PGDATA
to the legacy path makes pg18 find the existing cluster and upgrade it in place.
