Implement USER setting owner of COPY'd files.

The patchset actually implements the proposal docker#7537. It
adds the functionality for the `COPY` instruction to `chown` the
files it copies into the container to the current `USER` set in the
`Dockerfile`. It also adds tests to ensure that this UX change does
not regress in later revisions.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)