Use standard alpine image as base for mailman-web

This way we can depend on alpine's python3 package which is binary
compatible with the rest of the python packages in the distribution.

This requires spelling out python3 explicitly in several places since
alpine still defaults to python 2 for /usr/bin/python.

web/mailman-web/manage.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 import os
 import sys