Stop using dockerfile/* images

As per
http://blog.docker.com/2015/03/updates-available-to-popular-repos-update-your-images/
docker has stopped answering dockerfile/redis and dockerfile/nginx.  Fix all
users in our tree.  Sadly this means a lot of published examples are now broken.

docs/getting-started-guides/pod.json

@@ -8,7 +8,7 @@
       "id": "php",
       "containers": [{
         "name": "nginx",
-        "image": "dockerfile/nginx",
+        "image": "nginx",
         "ports": [{
           "containerPort": 80,
           "hostPort": 8081