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.
I think it's still open whether we support the environment variables or not. They are problematic, and I don't believe the enumeration they provide is useful.