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>
The <dest> (avoid starting sentences with lower case)