patch postgres mount location/type

this stops the db instances from being anonymous and from not
persevering between compose up-downs (i think?)

compose.yaml

@@ -21,7 +21,7 @@ services:
     ports:
       - 5432:5432
     volumes:
-      - pg_volume:/var/lib/postgresql/data:rw
+      - pg_volume:/var/lib/postgresql
     stop_grace_period: 120s
     environment:
       POSTGRES_USER: mnemo