Reference dockerfile used for zulip-postgres

flux2/apps/zulip/zulip-values-configmap.yaml

@@ -63,6 +63,8 @@ data:
 
     postgresql:
       image:
+        # Dockerfile used for building the image is at
+        # https://github.com/zulip/zulip/pull/21259
         repository: open.greenhost.net:4567/stackspin/stackspin/zulip-postgresql
         tag: 14
       persistence: