new: change postgres image towards the classical postgres

This image is based on ubuntu, and the charm supports pgm out of
the box. It doesn't need any docker-entrypoint. No internal/magical
knowledge.

compose.yml

@@ -4,7 +4,7 @@ odoo:
     admin-password: rantanplan
   relations:
     postgres-database:
-      postgres-alpine:
+      postgres:
         user: odoo
         dbname: odoo
     web-proxy: