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.

Closes #7537

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
Ok, i see it now.  This is not a bad UI and confusing because it's returning an error code when it does not do anything.  It should be PasswdFilePath() or similar so it does not look like we are reading the file.