Add some documentation for the container environment.

@smarterclayton @bgrant0607 @thockin @erictune 

We've talked about some of this in the past.  Mostly this describes what is already implemented, but there are some proposals in there as well, please take a look.
As of yesterday, we don't use the container port in the variables, so this should just be FOO_SERVICE_HOST and FOO_SERVICE_PORT, I think