Add example for usage with swarm-cronjob (#89)

README.md

@@ -12,7 +12,7 @@ A Docker swarm service for automatically updating your services whenever their b
                           --mount type=bind,source=/var/run/docker.sock,target=/var/run/docker.sock,ro \
                           mazzolino/shepherd
 
-## Or with docker-compose
+### Or with Docker Compose
 
     version: "3"
     services:
@@ -27,7 +27,7 @@ A Docker swarm service for automatically updating your services whenever their b
             constraints:
             - node.role == manager
 
-### Configuration
+## Configuration
 
 Shepherd will try to update your services every 5 minutes by default. You can adjust this value using the `SLEEP_TIME` variable.
 
@@ -109,6 +109,15 @@ Example:
                         --mount type=bind,source=/root/.docker/config.json,target=/root/.docker/config.json,ro \
                         mazzolino/shepherd
 
+## Running on a cron schedule
+
+When running shepherd as described with a `SLEEP_TIME`, the de facto running times will drift the longer the container is running. If you want to run shepherd on a fixed schedule instead, it is recommended to pair it with [swarm-cronjob](https://github.com/crazy-max/swarm-cronjob):
+
+1. Create a *swarm-cronjob* service [as described in its documentation](https://crazymax.dev/swarm-cronjob/install/docker/#usage).
+2. Set `RUN_ONCE_AND_EXIT` to `true`, `replicas` to `0` and `restart_policy` to `condition: none`. Add docker labels for the schedule.
+
+See [docker-compose.swarm-cronjob.yml](docker-compose.swarm-cronjob.yml) for a full stack example which includes both shepherd as well as swarm-cronjob.
+
 ## How does it work?
 
 Shepherd just triggers updates by updating the image specification for each service, removing the current digest.

docker-compose.swarm-cronjob.yml

@@ -0,0 +1,38 @@
+version: "3"
+
+services:
+  app:
+    build: .
+    image: mazzolino/shepherd
+    environment:
+      TZ: 'US/Eastern'
+      FILTER_SERVICES: ''
+      IGNORELIST_SERVICES: "test"
+      RUN_ONCE_AND_EXIT: "true"
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock:ro
+    deploy:
+      replicas: 0
+      restart_policy:
+        condition: none
+      labels:
+        - swarm.cronjob.enable=true
+        # Start service every day at midnight
+        - swarm.cronjob.schedule=0 0 0 * * *
+        - swarm.cronjob.skip-running=true
+      placement:
+        constraints:
+        - node.role == manager
+
+  scheduler:
+    image: crazymax/swarm-cronjob:latest
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock:ro
+    environment:
+      - "TZ=Europe/Berlin"
+      - "LOG_LEVEL=info"
+      - "LOG_JSON=false"
+    deploy:
+      placement:
+        constraints:
+          - node.role == manager