kubeadm: container runtime is by default docker in kubelet v1.23

- can be removed in v1.26

cmd/kubeadm/app/phases/kubelet/flags.go

@@ -76,6 +76,8 @@ func WriteKubeletDynamicEnvFile(cfg *kubeadmapi.ClusterConfiguration, nodeReg *k
 func buildKubeletArgMapCommon(opts kubeletFlagsOpts) map[string]string {
 	kubeletFlags := map[string]string{}
 	kubeletFlags["container-runtime-endpoint"] = opts.nodeRegOpts.CRISocket
+	// container runtime is by default docker in kubelet v1.23, so it can be removed in v1.26
+	kubeletFlags["container-runtime"] = "remote"
 
 	// This flag passes the pod infra container image (e.g. "pause" image) to the kubelet
 	// and prevents its garbage collection

cmd/kubeadm/app/phases/kubelet/flags_test.go

@@ -40,6 +40,7 @@ func TestBuildKubeletArgMap(t *testing.T) {
 				},
 			},
 			expected: map[string]string{
+				"container-runtime":          "remote",
 				"container-runtime-endpoint": "unix:///var/run/containerd/containerd.sock",
 				"hostname-override":          "override-name",
 			},
@@ -65,6 +66,7 @@ func TestBuildKubeletArgMap(t *testing.T) {
 				registerTaintsUsingFlags: true,
 			},
 			expected: map[string]string{
+				"container-runtime":          "remote",
 				"container-runtime-endpoint": "unix:///var/run/containerd/containerd.sock",
 				"register-with-taints":       "foo=bar:baz,key=val:eff",
 			},
@@ -78,6 +80,7 @@ func TestBuildKubeletArgMap(t *testing.T) {
 				pauseImage: "k8s.gcr.io/pause:3.7",
 			},
 			expected: map[string]string{
+				"container-runtime":          "remote",
 				"container-runtime-endpoint": "unix:///var/run/containerd/containerd.sock",
 				"pod-infra-container-image":  "k8s.gcr.io/pause:3.7",
 			},