[Postfix] Allow to create an extra.cf file besides main.cf to merge overrides

[Postfix] Better logging while bootstrapping
[Postfix] Stop container, when a service fails (easier to monitor)

data/Dockerfiles/postfix/supervisord.conf

@@ -1,4 +1,5 @@
 [supervisord]
+pidfile=/var/run/supervisord.pid
 nodaemon=true
 user=root
 
@@ -12,6 +13,10 @@ autostart=true
 
 [program:postfix]
 command=/opt/postfix.sh
+stdout_logfile=/dev/stdout
+stdout_logfile_maxbytes=0
+stderr_logfile=/dev/stderr
+stderr_logfile_maxbytes=0
 autorestart=true
 
 [eventlistener:processes]