Allow for relative paths on ADD/COPY

Moved Tianon's PR from: https://github.com/docker/docker/pull/7870
on top of the latest code

Closes: #3936

Signed-off-by: Doug Davis <dug@us.ibm.com>
Signed-off-by: Andrew Page <admwiggin@gmail.com>
As before, avoid starting sentences with code samples, please. See style guide: https://docs.docker.com/contributing/docs_style-guide/#capitalization