Fix build-docker.sh bug (#515)

* Build examples by default

The [documentation](http://ray.readthedocs.io/en/latest/install-on-docker.html#build-docker-images)  says that the `examples` image is built by `build-docker.sh` script.

* Update build-docker.sh

build-docker.sh

@@ -45,6 +45,8 @@ rm ./docker/deploy/ray.tar ./docker/deploy/git-rev
 if [ ! $SKIP_EXAMPLES ]; then
     if [ $OUTPUT_SHA ]; then
         IMAGE_SHA=$(docker build $NO_CACHE -q -t ray-project/examples docker/examples)
+    else
+        docker build --no-cache -t ray-project/examples docker/examples
     fi
 fi