[autoscaler]: automatically pull new docker image (#6111)

* Docker: automatically pull new image

* Fix missing value in schema

* Address review comments

python/ray/autoscaler/aws/example-full.yaml

@@ -25,6 +25,9 @@ autoscaling_mode: default
 docker:
     image: "" # e.g., tensorflow/tensorflow:1.5.0-py3
     container_name: "" # e.g. ray_docker
+    # If true, pulls latest version of image. Otherwise, `docker run` will only pull the image
+    # if no cached version is present.
+    pull_before_run: True
     run_options: []  # Extra options to pass into "docker run"
 
     # Example of running a GPU head with CPU workers