Adds an example docker compose file that can be used to quickly
stand up an SMTP sink service that is useful when load testing
an SMTP server.
The sink has configurable and "realistic" bounce response rates,
so that you can excercise queue growth scenarios.
an earlier version of the file had redis set to global,
but since that is one copy per swarm node, it wouldn't
behave correctly and is now set to replicated with 1
replica.
Wez Furlong