Update "relative WORKDIR" logic to be slightly simpler

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Add support for relative ADD (which becomes relative to the current WORKDIR)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)