Optional conversion from AUFS to OverlayFS whiteout format in tar -> squashfs

[dtrudg]

If a standard OCI image, with tar layers, is converted to squashfs layer format then it is likely that the resulting squashfs layers will be combined into a container root filesystem using OverlayFS. This will be the case for SingularityCE.

The image-spec tar layer format specifies AUFS style whiteout/opaque markers for files/directories that have been deleted between layers:

• https://github.com/opencontainers/image-spec/blob/main/layer.md#whiteouts
• https://github.com/opencontainers/image-spec/blob/main/layer.md#whiteouts

These markers are not understood by OverlayFS, which uses different markers:

• https://docs.kernel.org/filesystems/overlayfs.html#whiteouts-and-opaque-directories

There is no practical way to convert the markers from AUFS -> OverlayFS format at mount time. Therefore, oci-tools must be able to convert from AUFS -> OverlayFS format in the tar -> squashfs conversion.