Add support for relative COPY/ADD (relative to WORKDIR to be consistent with the relative WORKDIR support)

Signed-off-by: Andrew Page <admwiggin@gmail.com>
add docs for relative paths to ADD & COPY

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)