Add initial version

Dockerfile

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+FROM docker
+
+COPY shepherd /usr/local/bin/shepherd
+
+ENTRYPOINT ["/usr/local/bin/shepherd"]

README.md

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+# Shepherd
+
+A Docker swarm service for automatically updating your services whenever their base image is refreshed.
+
+## Usage
+
+    docker service create --name shepherd \
+                          --mount type=bind,source=/var/run/docker.sock,target=/var/run/docker.sock,ro \
+                          mazzolino/shepherd
+
+Shepherd will try to update your services every 5 minutes.
+
+## How does it work?
+
+Shepherd just triggers updates by updating the image specification for each service, removing the current digest.
+
+Most of the work is thankfully done by Docker which [resolves the image tag, checks the registry for a newer version and updates running container tasks as needed](https://docs.docker.com/engine/swarm/services/#update-a-services-image-after-creation).
+
+Also, Docker handles all the work of [applying rolling updates](https://docs.docker.com/engine/swarm/swarm-tutorial/rolling-update/). So at least with replicated services, there should be no noticeable downtime.

docker-compose.yml

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+version: "3"
+
+services:
+  app:
+    build: .
+    image: mazzolino/shepherd
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock

shepherd

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+#!/bin/ash
+# shellcheck shell=dash
+set -euo pipefail
+
+update_services() {
+  for service in $(IFS="\n" docker service ls --quiet); do
+    local name image
+    name="$(docker service inspect "$service" -f '{{.Spec.Name}}')"
+    image_with_digest="$(docker service inspect "$service" -f '{{.Spec.TaskTemplate.ContainerSpec.Image}}')"
+    image=$(echo "$image_with_digest" | cut -d@ -f1)
+
+    echo "Updating service $name with image $image"
+    docker service update "$service" --image "$image" > /dev/null
+  done
+}
+
+main() {
+  while true; do
+    update_services
+    sleep 5m
+  done
+}
+
+main "$@"