Ensure Watchtower removes old images

The github runner machine ran our of disk space because of old runner images.
This should ensure that doesn't happen any more :o)

Signed-off-by: Matt Spencer matthew@thespencers.me.uk

github-runner.yaml

@@ -27,6 +27,8 @@
         name: watchtower
         image: containrrr/watchtower
         restart_policy: always
+        command:
+          - --cleanup
         volumes:
           - /var/run/docker.sock:/var/run/docker.sock
         state: started