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"]