Change default container runtime from docker to containerd

[afbjorklund]

This is a part of #13250, which makes the runtime dynamic

Since Kubernetes is only tested using the "containerd" runtime, it makes sense for minikube to default to using this runtime.

It is also possible to use the "docker" runtime or the "crio" runtimes, but these are not tested by Kubernetes before release.

So the suggestion is to change the new default, for k8s 1.24

---

This is a more specific change than the old "MEP" proposal

• MEP: CRI: Containerd by default, Buildkitd on demand #9639

It is already a part of the PR, but noted here for reference...

• Make the default container runtime dynamic #13251

// defaultRuntime returns the default container runtime
func defaultRuntime(k8sVersion string) string {
       // minikube default
       return constants.Docker
}

// defaultRuntime returns the default container runtime
func defaultRuntime(k8sVersion string) string {
	version, _ := util.ParseKubernetesVersion(k8sVersion)
	// since dockershim removal, only containerd is tested
	if version.GTE(semver.MustParse("1.24.0-alpha.1")) {
		return constants.Containerd
	}
	// default dockershim
	return constants.Docker
}

But it doesn't have to be tied to the dockershim removal at all.

It could be changed for k8s 1.23 too, or it could wait until k8s 1.25 ?

[afbjorklund]

This change is being pushed by upstream, while blaming dockershim:

• Dockershim removal feedback & issues kubernetes#106917

We have already seen some breakage, due to Docker not being tested:

• Kubernetes versin v1.23.0-rc.0 breaks crictl images list #13077

[afbjorklund]

Note that versions before Kubernetes 1.24 will still default to dockershim.

• KEP-2221: Removing dockershim from kubelet

And that versions after Kubernetes 1.24 can still choose to use Docker (opt-in).

• Start cri-docker service automatically, if using docker and kubernetes version >= 1.24 #12099

		sp := c.Socket // InternalDockerCRISocket
		cs := ""
		// There is no more dockershim socket, in Kubernetes version 1.24 and beyond
		if sp == "" && c.KubernetesVersion.GTE(semver.MustParse("1.24.0-alpha.1")) {
			sp = ExternalDockerCRISocket
			cs = "cri-docker.socket"
		}

[afbjorklund]

Maybe obvious, but moving to containerd breaks any "Docker Desktop" dreams.

• add new flag --no-kubernetes #12845

The user can use nerdctl which is similar-but-not-same, just like podman for crio.

• Add nerdctl command for users outside kubernetes #13252

[afbjorklund]

Looks like Docker will still be a supported and tested container runtime, after all.

• interested in integrating with minikube Mirantis/cri-dockerd#1 (comment)

It might even get automatically detected by kubeadm, even if that is "deprecated".

https://github.com/kubernetes/kubernetes/blob/master/cmd/kubeadm/app/util/runtime/runtime.go

		"unix:///run/containerd/containerd.sock",
		"unix:///var/run/crio/crio.sock",
		"unix:///var/run/cri-dockerd.sock",

[afbjorklund]

Missing this usage information:

• Add popularity contest for different drivers and runtimes #8175
• Investigate CNCF processes to collect minikube usage metrics #11373

Upstream usage information:

• https://kubernetes.io/blog/2022/01/07/kubernetes-is-moving-on-from-dockershim/
• https://www.datadoghq.com/container-report/#8