Upgrade Postfix to 3.3, supporting foreground start

postfix/Dockerfile

@@ -5,8 +5,10 @@ RUN \
  # Create OS user (which will be picked up later by apk add)
  addgroup -S -g 587 postfix \
  && adduser -S -u 587 -h /var/spool/postfix -s /bin/false -g postfix -G postfix postfix \
- # Install Postfix
- && apk --no-cache add ca-certificates postfix s6 \
+ # Add edge/main repository
+ && echo '@edge http://dl-cdn.alpinelinux.org/alpine/edge/main' >>/etc/apk/repositories \
+ # Install Postfix 3.3
+ && apk --no-cache add ca-certificates postfix@edge s6 \
  # Build aliases database
  && newaliases \
  # Cleanup

postfix/docker/etc/services.d/postfix/run

@@ -1,7 +1,3 @@
-#!/bin/sh
+#!/bin/execlineb -P
 
-/usr/sbin/postfix -c /etc/postfix start
-
-while kill -0 $(cat /var/spool/postfix/pid/master.pid) 2>/dev/null; do
-    sleep 1
-done
+/usr/sbin/postfix -c /etc/postfix start-fg