builder/docker: upload files with `-a` flag when upstream bug is fixed

_This issue was originally opened by @mwhooker as hashicorp/packer#5409. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

We should set ownership of files based on the container user. In theory, this should be doable with the `-a` flag to `docker cp`, but there's a bug that prevents that. When https://github.com/moby/moby/issues/34142 is fixed, let's change `docker cp` to be `docker cp -a`