Add an MX record for example.com
Make postfix accept @<hostname> plus @example.com
Monkey-patch getpass() for automated session.txt
Run session inside of Docker, not Mininet
Fix run_session.py to run inside container

Only the locale C.UTF-8 exists inside our simple Ubuntu containers, not
en_US.UTF-8, and the PYTHONPATH no longer needs to point to a virtualenv
because we install Python packages system-wide.
Tweak and re-run session.txt in new playground