gitlab_runner: Add image prune cron job task to playbook

playbooks/gitlab_runner.yml

@@ -25,3 +25,11 @@
     - gitlab_runner
   environment:
     PYTHONPATH: /opt/ansible-dependencies/lib/python{{ env_ansible_deps_python_version | default(2.7) }}/site-packages
+  tasks:
+    - name: Install docker image prune crontab
+      cron:
+        name: "Prune unused docker images"
+        minute: "0"
+        hour: "*/4"
+        job: "docker image prune -f"
+      become: true