Don't check for docker.io prefix in docker preload

Right now, preload isn't working because it thinks that the
kubernetes/dashboard images don't exist. This is because they now have a
docker.io prefix introduced by 4c54e78681

Since `docker images` won't return image names with that prefix, we want
to strip the images of `docker.io` before checking if they exists in the
docker daemon.

This also fixes the TestPause/serial/SecondStartNoReconfiguration
failing integration test.

pkg/minikube/cruntime/docker.go

@@ -375,6 +375,7 @@ func dockerImagesPreloaded(runner command.Runner, images []string) bool {
 	}
 	preloadedImages := map[string]struct{}{}
 	for _, i := range strings.Split(rr.Stdout.String(), "\n") {
+		i = trimDockerIO(i)
 		preloadedImages[i] = struct{}{}
 	}
 
@@ -382,6 +383,7 @@ func dockerImagesPreloaded(runner command.Runner, images []string) bool {
 
 	// Make sure images == imgs
 	for _, i := range images {
+		i = trimDockerIO(i)
 		if _, ok := preloadedImages[i]; !ok {
 			klog.Infof("%s wasn't preloaded", i)
 			return false
@@ -390,6 +392,13 @@ func dockerImagesPreloaded(runner command.Runner, images []string) bool {
 	return true
 }
 
+// Remove docker.io prefix since it won't be included in images names
+// when we call 'docker images'
+func trimDockerIO(name string) string {
+	name = strings.TrimPrefix(name, "docker.io/")
+	return name
+}
+
 func dockerBoundToContainerd(runner command.Runner) bool {
 	// NOTE: assumes systemd
 	rr, err := runner.RunCmd(exec.Command("sudo", "systemctl", "cat", "docker.service"))