[autoscaler] Docker Support (#1505)

doc/source/autoscaling.rst

@@ -104,6 +104,15 @@ The ``example.yaml`` configuration is enough to get started with Ray, but for mo
     head_node:
         InstanceType: p2.8xlarge
 
+**Docker**: Specify docker image. This executes all commands on all nodes in the docker container,
+and opens all the necessary ports to support the Ray cluster.
+
+.. code-block:: yaml
+
+    docker:
+        image: tensorflow/tensorflow:1.5.0-py3
+        container_name: ray_docker
+
 **Mixed GPU and CPU nodes**: for RL applications that require proportionally more
 CPU than GPU resources, you can use additional CPU workers with a GPU head node.