Assign a $PORT environment variable to each process.
Assign a $PORT number for each process
This pull request raises a general question about the Shoreman project: is there a goal to be compatible with Foreman? Or was $PORT intentionally left out? I can see good reasons to not have $PORT.
It would be worth explaining in the README.md under a section entitled, "Foreman compatibility," about this project's Foreman compatibility goals.