Steps to reproduce:

• Generate a local docker archive (test.tar) that uses uncompressed tarballs as layers
• Run skopeo copy docker-archive:test.tar dir://out
• Observe that the manifest in out/manifest.json has layers specified as "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip"
• Observe that the actual layer files are uncompressed tarballs: e719c9bac8ba3f1aac6ab6d3dbd006be2bee3dca4030e0fca96297ba72bdb569/layer.tar: POSIX tar archive