add support for COPY to docker build

This adds a COPY command to docker build which works like ADD, but is
only for local files and it doesn't extract files.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #6089 from unclejack/docker_build_copy

add support for COPY to docker build