add example of immutable docker image digest

Author: mr-c

CommandLineTool.yml

@@ -813,7 +813,10 @@ $graph:
         "_type": "@vocab"
     - name: dockerPull
       type: string?
-      doc: "Specify a Docker image to retrieve using `docker pull`."
+      doc: |
+        Specify a Docker image to retrieve using `docker pull`. Can contain the
+        immutable digest to ensure an exact container is used:
+        `dockerPull: ubuntu@sha256:45b23dee08af5e43a7fea6c4cf9c25ccf269ee113168c19722f87876677c5cb2`
     - name: dockerLoad
       type: string?
       doc: "Specify a HTTP URL from which to download a Docker image using `docker load`."