new: added ``cron`` service, ``letsencrypt`` uses it for renewing certificates.

compose.yml

@@ -100,6 +100,17 @@ letsencrypt:
     #       key: XXX
     #       secret: YYY
     #     consumer_key: ZZZ
+  relations:
+    schedule-command:
+      cron:   ## will schedule renew of certs every week
+        schedule: "30 3 * * 7" ## min hour day-of-month month day-of-week
+
+
+## Cron is present (even if empty) to add it automatically to any
+## ``compose up`` (without arguments).
+cron:
+
+
 # mattermost:
 #   charm: mattermost
 #   relations: