build: agent is the image's default entrypoint; drop the old server examples (#7)

deploy/agent.compose.yml

@@ -0,0 +1,19 @@
+# Example agent deployment, one per machine. The master UI generates this
+# snippet with MASTER_URL, MASTER_KEY and AGENT_TOKEN filled in (Agents -> Add
+# agent); the token is shown once.
+services:
+  updater-agent:
+    image: gitea.shcizo.se/shcizo/package-updater:latest
+    entrypoint: ["/usr/local/bin/updater-agent"]
+    restart: unless-stopped
+    environment:
+      MASTER_URL: wss://10.0.0.5:8090/agent
+      MASTER_KEY: sha256:<fingerprint shown in the master UI>
+      AGENT_TOKEN: <token shown once when the agent was added>
+      UPDATE_TIMEOUT: 5m
+      # Optional registry credentials (a config.json written by docker login):
+      # DOCKER_CONFIG: /docker-config
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock
+      # - /path/to/stacks:/path/to/stacks  # mount each stacks directory at the same path, read-write
+      # - /root/.docker/config.json:/docker-config/config.json:ro