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.
Seems like we should allow exec handler for this.  There should be a way for us to hack it so the handler runs after the container environment is setup but before the docker CMD runs.