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.
Hooks have been proposed for Docker (https://github.com/docker/docker/issues/6982) and we will have events, so I don't think playing up the contrast is useful. Instead, I'd refer to systems that DO provide similar hooks, such as the ones I enumerated in https://github.com/GoogleCloudPlatform/kubernetes/issues/140.