Add Dockerfile for vitess/root image.

docker/root/Dockerfile

@@ -0,0 +1,6 @@
+# This is an image that just changes the default user to root.
+# It's useful for cases when the 'docker run --user' flag can't be used,
+# for example when running containers through a scheduler like Kubernetes.
+FROM vitess/base
+
+USER root