Directly run the master runner as CMD, instead of mailman start.

`mailman start` command does not nothing but parses the configuration and starts
the appropriate runners. The start command is implemented such that it forks the
master runner, which in-turn starts the all the rest of the runners. It turns
out that `master` is also exported as an console script which can directly be
started instead of running `mailman start` command. This makes containerizing
mailman much more easier.

core/Dockerfile

@@ -19,3 +19,5 @@ WORKDIR /opt/mailman
 EXPOSE 8001
 
 ENTRYPOINT ["/opt/run.sh"]
+
+CMD ["/usr/local/bin/master", "-C",  "/config/mailman.cfg"]

core/assets/run.sh

@@ -117,21 +117,4 @@ api_key: $HYPERKITTY_API_KEY
 EOF
 
 
-# SIGTERM Handler so that the container shuts down gracefully.
-function term_handler () {
-	echo "Stopping mailman ..."
-	mailman stop
-	exit 143 # SIGTERM
-}
-
-# Set the trap for SIGTERM.
-trap 'kill ${!}; term_handler' SIGTERM
-
-# Start the mailman server. Mailman will start the master runner and then exit.
-mailman -C /config/mailman.cfg start --force &
-
-# wait forever.
-while true
-do
-  tail -f /dev/null & wait ${!}
-done
+exec "$@"