Commands in the pipeline should either implement or pass-through command messages. This amounts to a proof-of-concept implementation of the "pipeline" design of Docker plugins. Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes) Upstream-commit: ed62ca5b2f673fde54c2ffc5edaf0e5bcbd1b721 Component: engine