Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
This `/bin/sh -c #(nop) ...` is a great example of a Docker implementation detail that could be nailed down here. It's [used as a hack in the builder](https://github.com/docker/docker/blob/64ebffe7b6e4a6732b3b6f56585bc8d01f4f8ed0/builder/internals.go#L67) so that intermediate build steps remember the last command that was run, even when a _real_ `CMD` was previously specified.