Make default container runtime dynamic

[afbjorklund]

Since Kubernetes 1.24, there is no longer a "default" container runtime (since the removal of dockershim).

• https://kubernetes.io/blog/2020/12/02/dont-panic-kubernetes-and-docker/

• https://kubernetes.io/blog/2021/11/12/are-you-ready-for-dockershim-removal/

Instead, the user will have to supply a container runtime. But the only tested container runtime is containerd.

---

The current constant should be converted, into a default placeholder - which translates into a dynamic value

        // Containerd is the default name and spelling for the containerd container runtime
        Containerd = "containerd"
        // CRIO is the default name and spelling for the cri-o container runtime
        CRIO = "crio"
        // DefaultContainerRuntime is our default container runtime
        DefaultContainerRuntime = "docker"

A new constant Docker should be introduced instead, to replace all the hardcoded "docker" strings...

User can still choose it as the container runtime (no matter the default), using --container-runtime=docker.

Minikube should still provide a default container runtime for each k8s version, even if Kubernetes does not.

--container-runtime='': The container runtime to be used. Valid options: docker, cri-o, containerd (default: auto)

https://kubernetes.io/docs/setup/production-environment/container-runtimes/

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[afbjorklund]

Suggested change:

        // Containerd is the default name and spelling for the containerd container runtime
        Containerd = "containerd"
        // CRIO is the default name and spelling for the cri-o container runtime
        CRIO = "crio"
        // Docker is the default name and spelling for the docker container runtime
        Docker = "docker"
        // DefaultContainerRuntime is our default container runtime
        DefaultContainerRuntime = ""

Then there needs to be code handling this, currently there's a truckload of viper.GetString(containerRuntime)

In the future, the value will needs to be converted into an actual runtime and the default computed by k8s version.

[afbjorklund]

Note that you can still use docker or crio, they are just not getting tested upstream (but with Mirantis or with OpenShift)

container runtime	container engine	CRI	Socket
"containerd"	nerdctl	(cri-containerd) containerd (with cri plugin)	unix:///run/containerd/containerd.sock
"docker"	docker	cri-dockerd (dockershim)	unix:///var/run/cri-docker.sock (1.24-) unix:///var/run/dockershim.sock (-1.23)
"crio"	podman	cri-o (one version per k8s)	unix:///var/run/crio/crio.sock

Contrary to popular belief, the removal of dockershim is not the same as the removal of docker. Just enforcing an API...

Starting with Kubernetes 1.24, the old Docker-specific hacks have been removed from the codebase. Instead use CRI.

• Start cri-docker service automatically, if using docker and kubernetes version >= 1.24 #12099
• Use cri-dockerd instead of dockershim where needed #12102