Add container spec

This document specifies the current container specification with all the
underlying setup and configuration that goes into making a container.
This is the v1 spec that is today but allows for flexability when we
introduce more features such as seccomp default profiles and more.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
We should probably mention the effects on rootfs after a container run, outside what the app does. eg. libcontainer unmounts everything it mounts as part of container config on destruction.