[Dovecot] Use events to kill supervisord when main proc dies

[Postfix] Use events to kill supervisord when main proc dies
[SOGo] Use events to kill supervisord when main proc dies

data/Dockerfiles/postfix/stop-supervisor.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+
+printf "READY\n";
+
+while read line; do
+  echo "Processing Event: $line" >&2;
+  kill -3 $(cat "/var/run/supervisord.pid")
+done < /dev/stdin