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