Added support for multiple endpoints in X-Docker-Endpoints header
Docker-DCO-1.1-Signed-off-by: Joffrey F <joffrey@docker.com> (github: shin-)
[Documentation - API] Add missing 'signal' parameter for /containers/:id/kill endpoint

Docker-DCO-1.1-Signed-off-by: Leo Unbekandt <leo.unbekandt@appsdeck.eu> (github: Soulou)
engine.Len returns the number of keys in an env

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #4576 from Soulou/4549-doc-kill-signal-param

Add missing 'signal' parameter for kill endpoint
Merge pull request #5199 from shin-/registry_mirrors_support

Added support for multiple endpoints in X-Docker-Endpoints header
Merge pull request #5431 from shykes/engine-len

engine.Len returns the number of keys in an env