rootless: support --container-runtime=docker

This allows running Kubernetes with (CRI-)Dockerd in Rootless Docker:
```
minikube start --rootless --driver=docker --container-runtime=docker
```

It is still recommended to use `--container-runtime=containerd`.

Fix issue 17322

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

cmd/minikube/cmd/start_flags.go

@@ -619,9 +619,6 @@ func generateNewConfigFromFlags(cmd *cobra.Command, k8sVersion string, rtime str
 		}
 		if si.Rootless {
 			out.Styled(style.Notice, "Using rootless {{.driver_name}} driver", out.V{"driver_name": driver.FullName(drvName)})
-			if cc.KubernetesConfig.ContainerRuntime == constants.Docker {
-				exit.Message(reason.Usage, "--container-runtime must be set to \"containerd\" or \"cri-o\" for rootless")
-			}
 			// KubeletInUserNamespace feature gate is essential for rootless driver.
 			// See https://kubernetes.io/docs/tasks/administer-cluster/kubelet-in-userns/
 			cc.KubernetesConfig.FeatureGates = addFeatureGate(cc.KubernetesConfig.FeatureGates, "KubeletInUserNamespace=true")

pkg/minikube/cruntime/docker.go

@@ -134,9 +134,15 @@ func (r *Docker) Active() bool {
 // Enable idempotently enables Docker on a host
 func (r *Docker) Enable(disOthers bool, cgroupDriver string, inUserNamespace bool) error {
 	if inUserNamespace {
-		return errors.New("inUserNamespace must not be true for docker")
+		if err := CheckKernelCompatibility(r.Runner, 5, 11); err != nil {
+			// For using overlayfs
+			return fmt.Errorf("kernel >= 5.11 is required for rootless mode: %w", err)
+		}
+		if err := CheckKernelCompatibility(r.Runner, 5, 13); err != nil {
+			// For avoiding SELinux error with overlayfs
+			klog.Warningf("kernel >= 5.13 is recommended for rootless mode %v", err)
+		}
 	}
-
 	if disOthers {
 		if err := disableOthers(r, r.Runner); err != nil {
 			klog.Warningf("disableOthers: %v", err)

site/content/en/docs/drivers/docker.md

@@ -49,7 +49,7 @@ minikube start --driver=docker --container-runtime=containerd
 Unlike Podman driver, it is not necessary to set the `rootless` property of minikube (`minikube config set rootless true`).
 When the `rootless` property is explicitly set but the current Docker host is not rootless, minikube fails with an error.
 
-The `--container-runtime` flag must be set to "containerd" or "cri-o". "containerd" is recommended.
+It is recommended to set the `--container-runtime` flag to "containerd".
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