Add Heroku comment

Author: penyuan

Dockerfile

@@ -1,6 +1,9 @@
 # SPDX-FileCopyrightText: 2021 Pen-Yuan Hsing
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
+# Container image following Heroku recommendations: 
+# https://devcenter.heroku.com/articles/container-registry-and-runtime#testing-an-image-locally
+
 FROM python:3.10-slim-bullseye
 
 WORKDIR /opt/app