api/container.go: an API-specific representation of a container

This breaks the dependency from the remote API implementation to the
internal representation of a container. Instead it uses its own partial
representation of a container, with only required fields.

* This preserves reverse-compatibility with all past implementations of the remote API.

* This clarifies which fields are guaranteed to be present in a response
	A docker remote api server *may* return more fields in a Container
	object, but their presence and semantics are not guaranteed and should
	not be relied upon by client implementations.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)