Support multiple ports per service/container

[cfregly]

A lot of data-focused services use multiple ports from a single Docker container including 1/ public communication, 2/ internal communication between containers, and 3/ metrics scraping, etc.

It would be nice if I could specify a list of ports that the Docker image will EXPOSE. Some will be public, some will be internal (between service containers), some will be internally-retrieved from another service (ie. metrics aggregator).