Actually determine the relative path of the Dockerfile to the context
directory. Error out if the relative path starts with "../".
Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Upstream-commit: af06d289e4300f3b86ec9deba7280c3c37fa4e05
Component: engine