Support `user.rootlesscontainers` xattr in containerd snapshotter (and Docker/Moby graphdriver)

[AkihiroSuda]

subuidless/README.md

Line 74 in 6e36c0c

The UID ang GID are recorded to [the `user.rootlesscontainers` xattr](https://github.com/rootless-containers/proto) of the target file.

user.rootlesscontainers xattr should be supported in containerd snapshotter (and Docker/Moby graphdriver) to extract the container rootfs from the image