The `cri-in-userns` stage is for testing "CRI-in-UserNS", which should be used in conjunction with "Kubelet-in-UserNS":
https://github.com/kubernetes/enhancements/tree/master/keps/sig-node/2033-kubelet-in-userns-aka-rootless
This feature is mostly expected to be used for `kind` and `minikube`.
Requires Rootless Docker/Podman/nerdctl with cgroup v2 delegation: https://rootlesscontaine.rs/getting-started/common/cgroup2/
(Rootless Docker/Podman/nerdctl prepares the UserNS, so we do not need to create UserNS by ourselves)
Usage:
```
podman build --target cri-in-userns -t cri-in-userns -f contrib/Dockerfile.test .
podman run -it --rm --privileged cri-in-userns
```
The stage is tested on CI with Rootless Podman on Fedora 34 on Vagrant.
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
