Merge pull request #55515 from yguo0905/fix-gke-gci-script

Set CONTAINER_RUNTIME default value to 'docker'

cluster/gce/gci/configure-helper.sh

@@ -2050,7 +2050,7 @@ fi
 
 override-kubectl
 # Run the containerized mounter once to pre-cache the container image.
-if [[ "${CONTAINER_RUNTIME:-}" == "docker" ]]; then
+if [[ "${CONTAINER_RUNTIME:-docker}" == "docker" ]]; then
   assemble-docker-flags
 fi
 start-kubelet