chore: update docker in containerengine example

Currently we are pegged to Node:4. This is an older LTS image and
clocks in at 655 MB.

This PR updates us to Node:6-alpine. This will use a more modern
LTS branch, and also switches to the Alpine container which clocks
in at 54 MB.

containerengine/hello-world/Dockerfile

@@ -12,7 +12,7 @@
 # limitations under the License.
 
 # [START all]
-FROM node:4
+FROM node:6-alpine
 EXPOSE 8080
 COPY server.js .
 CMD node server.js