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.
The first thing I would have expected to see here would be something about my filesystem.  Maybe link to volumes.md, and or say something like "The filesystem seen by a process in a container is a combination of its Docker image and any Volumes it requested," and so on.