Merge pull request #2361 from hashicorp/f-docker-coordination

Docker Image Coordinator and caching

website/source/docs/drivers/docker.html.md

@@ -410,6 +410,11 @@ options](/docs/agent/configuration/client.html#options):
 * `docker.cleanup.image` Defaults to `true`. Changing this to `false` will
   prevent Nomad from removing images from stopped tasks.
 
+* `docker.cleanup.image.delay` A time duration that defaults to `3m`. The delay
+  controls how long Nomad will wait between an image being unused and deleting
+  it. If a tasks is received that uses the same image within the delay, the
+  image will be reused.
+
 * `docker.volumes.enabled`: Defaults to `true`. Allows tasks to bind host paths
   (`volumes`) inside their container and use volume drivers (`volume_driver`).
   Binding relative paths is always allowed and will be resolved relative to the