Add cron job to prune unused Docker data (close #168)

roles/docker/tasks/main.yml

@@ -27,6 +27,15 @@
       - docker
       - docker-compose
 
+- name: Configure cron job to prune unused Docker data weekly
+  cron:
+    name: Prune unused Docker data
+    cron_file: ansible_docker_prune
+    job: 'docker system prune -fa && docker volume prune -fa'
+    special_time: weekly
+    user: root
+    state: present
+
 - name: create folder structure for bind mounts
   file:
     name: "{{ volume_root_folder }}"