Add support for relative COPY/ADD

... (relative to WORKDIR to be consistent with the relative WORKDIR support)

Fixes #3936

(See also discussion on #4849)
I think you've got the gist of it, though.  The `<dest>` argument is a path within the image to where the content from `<src>` gets copied, and if it's relative, it's relative to `WORKDIR`.