make a default container runtime constant so that it is easy to change in the future

cmd/minikube/cmd/start_flags.go

@@ -142,7 +142,7 @@ func initMinikubeFlags() {
 	startCmd.Flags().String(kicBaseImage, kic.BaseImage, "The base image to use for docker/podman drivers. Intended for local development.")
 	startCmd.Flags().Bool(keepContext, false, "This will keep the existing kubectl context and will create a minikube context.")
 	startCmd.Flags().Bool(embedCerts, false, "if true, will embed the certs in kubeconfig.")
-	startCmd.Flags().String(containerRuntime, "docker", fmt.Sprintf("The container runtime to be used (%s).", strings.Join(cruntime.ValidRuntimes(), ", ")))
+	startCmd.Flags().String(containerRuntime, constants.DefaultContainerRuntime, fmt.Sprintf("The container runtime to be used (%s).", strings.Join(cruntime.ValidRuntimes(), ", ")))
 	startCmd.Flags().Bool(createMount, false, "This will start the mount daemon and automatically mount files into minikube.")
 	startCmd.Flags().String(mountString, constants.DefaultMountDir+":/minikube-host", "The argument to pass the minikube mount command on start.")
 	startCmd.Flags().StringSliceVar(&config.AddonList, "addons", nil, "Enable addons. see `minikube addons list` for a list of valid addon names.")

pkg/minikube/constants/constants.go

@@ -45,6 +45,8 @@ const (
 	RegistryAddonPort = 5000
 	// CRIO is the default name and spelling for the cri-o container runtime
 	CRIO = "crio"
+	// DefaultContainerRuntime is our default container runtime
+	DefaultContainerRuntime = "docker"
 
 	// APIServerName is the default API server name
 	APIServerName = "minikubeCA"

test/integration/main_test.go

@@ -27,6 +27,7 @@ import (
 	"testing"
 	"time"
 
+	"k8s.io/minikube/pkg/minikube/constants"
 	"k8s.io/minikube/pkg/minikube/driver"
 )
 
@@ -133,7 +134,7 @@ func KicDriver() bool {
 func ContainerRuntime() string {
 	flag := "--container-runtime="
 	if !strings.Contains(*startArgs, flag) {
-		return "docker"
+		return constants.DefaultContainerRuntime
 	}
 	for _, s := range StartArgs() {
 		if strings.HasPrefix(s, flag) {