service: Allow custom postgres image

roles/service/tasks/database.yaml

@@ -4,7 +4,7 @@
     name: container
   vars:
     container_name: "{{ service_name }}-{{ service_database_type }}" # This doesn't use _service_database_name to allow container role handlers to work
-    container_image: "docker.io/library/postgres:{{ service_postgres_tag }}"
+    container_image: "{{ service_postgres_image }}:{{ service_postgres_tag }}"
     container_mounts:
       - type: volume
         source: "{{ _service_database_name }}"