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)
@crosbymichael The function names PR has been merged to libcontainer, but it probably won't show up here for quite a while (I tried to open an update PR, but there were a bunch of APIs that have been changed in libcontainer -- and I'm not sure how to update them).