[#812] Add docker.sock volume to the Scheduler

It's needed for the Scheduler to be able to run Docker containers (with the
`LocalExecutor`, the scheduler runs the tasks itself).

opentrials/opentrials#812

ansible/files/docker-cloud.yml

@@ -21,6 +21,8 @@ webserver:
 scheduler:
   image: 'opentrials/opentrials-airflow:latest'
   restart: always
+  volumes:
+    - '/var/run/docker.sock:/var/run/docker.sock'
   environment:
     - EXECUTOR
     - AIRFLOW_ENABLE_AUTH