unpack: doesn't preserve mtime

[cyphar]

This is because the OCI tooling doesn't appear to handle this properly (image.CreateRuntimeLayoutBundle is to blame). It will probably be a good idea to from-scratch implement this feature inside umoci so that there is more than one implementation of this unpacking functionality.

% umoci unpack --image opensuse --ref latest --bundle bundle1
% sleep 30s
% umoci unpack --image opensuse --ref latest --bundle bundle2
% gomtree -f bundle1/*.mtree -p bundle2/rootfs >/dev/null
% echo $?
1

Where all of the gomtree failed keywords are related to time.

[cyphar]

Opened a bug report upstream. opencontainers/image-tools#74

[cyphar]

This has been fixed in 7654ebc. In addition, I dealt with the issue of . not having entries in most images (we just use the Unix epoch as the atime and mtime to make sure that bundles are completely consistent).