Support copying files to a container with a given ownership #605

The Docker API for copying an archive to a container has a flag CopyUIDGID.

You could therefore chown a file locally to whatever UID/GID required, and this ownership will be retained in the container. This simulates an equivalent of

COPY --chown=user:group myfile /path/in/container/myfile

# OR

COPY myfile /path/in/container/myfile
RUN chown user:group myfile