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/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.6-alpine3.7
+FROM alpine:3.8
 
 MAINTAINER Abhilash Raj
 
@@ -12,11 +12,12 @@ COPY docker-entrypoint.sh /usr/local/bin/
 # rights for management script
 RUN set -ex \
 	&& apk add --no-cache --virtual .build-deps gcc libc-dev linux-headers \
-	   	postgresql-dev mariadb-dev \
+		postgresql-dev mariadb-dev python3-dev \
 	&& apk add --no-cache --virtual .mailman-rundeps bash sassc \
 	   postgresql-client mysql-client py-mysqldb curl mailcap \
-	&& pip install -U django==2.1.5 pip \
-	&& pip install mailmanclient==3.2.1 \
+	   python3 py3-setuptools \
+	&& python3 -m pip install -U django==2.1.5 pip \
+	&& python3 -m pip install mailmanclient==3.2.1 \
 		postorius==1.2.2 \
 		hyperkitty==1.2.1 \
 		django-mailman3==1.2.0 \
@@ -43,3 +44,5 @@ STOPSIGNAL SIGINT
 
 ENTRYPOINT ["docker-entrypoint.sh"]
 CMD ["uwsgi", "--ini", "/opt/mailman-web/uwsgi.ini"]
+
+ENV PYTHONPATH=/usr/lib/python3.6/site-packages