v2 ping now checks for a Docker-Distribution-API-Version
header that identifies the endpoint as "registry/2.0"
Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Upstream-commit: f46923be8eba81dd9e261554128b26a1c83ce398
Component: engine