community/rootlesskit: move from testing

I have been using nerdctl rootless for a couple of weeks. Since nerdctl recently got into communnity repo, I think including rootlesskit for 3.17 release would be a nice addition.

For anyone wondering how to set it up on Alpine, I wrote a blog about my own process: https://www.folliehiyuki.com/posts/rootfull-rootless-containers-on-btrfs-zfs/ (sorry for the self-advertisement 😄).