Warn if docker buildx is not available

Signed-off-by: Davanum Srinivas <davanum@gmail.com>

hack/lib/util.sh

@@ -707,14 +707,14 @@ function kube::util::ensure-cfssl {
   popd > /dev/null || return 1
 }
 
-# kube::util::ensure_dockerized
-# Confirms that the script is being run inside a kube-build image
+# kube::util::ensure-docker-buildx
+# Check if we have "docker buildx" commands available
 #
-function kube::util::ensure_dockerized {
-  if [[ -f /kube-build-image ]]; then
+function kube::util::ensure-docker-buildx {
+  if docker buildx >/dev/null 2>&1; then
     return 0
   else
-    echo "ERROR: This script is designed to be run inside a kube-build container"
+    echo "ERROR: docker buildx not available. Docker 19.03 or higher is required with experimental features enabled"
     exit 1
   fi
 }