Include UNIX permission bits in checksum computation

[MShekow]

While the caching mechanism of OCI image builders like BuildKit ignore the owner of files, they do account for the UNIX permission bits (r/w/x). Thus, directory-checksum should also account for these bits.

A possible solution is to add the permission bits to the end of the checksum, essentially treating it like being part of the checksum.

Experiments need be done whether this also applies to directories.