service: Allow custom postgres image

roles/service/meta/argument_specs.yaml

@@ -182,11 +182,17 @@ argument_specs:
           - postgres
           - none
         default: none
+      service_postgres_image:
+        description: Postgresql image to use.
+        type: str
+        required: false
+        default: docker.io/library/postgres
       service_postgres_tag:
         description:
           - Postgresql version to use.
           - Can be debian (n) or alpine-based (n-alpine), where n can be major version like 14 or minor like 14.13.
           - Required if service_database_type is postgres, does nothing otherwise
+          - If a custom postgres image is specified, see that image documentation for supported tags.
         type: str
         required: false
       service_redis: