Initial Dockge image update checker

.codex/project.md

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+# Project
+
+Name: dockge-image-update-checker
+
+Description: Companion service that checks Dockge stack images for newer registry digests.
+
+Stack: Node.js 20+, Docker, Docker Compose
+
+## Commands
+
+```bash
+npm test
+npm run build
+npm run check
+```
+
+## Runtime
+
+Environment variables:
+
+- `STACKS_DIR`: Dockge stacks directory. Defaults to `/opt/stacks`.
+- `PORT`: HTTP port. Defaults to `8080`.
+- `CHECK_INTERVAL_SECONDS`: background refresh interval. Defaults to `3600`.
+- `DOCKER_SOCKET`: Docker socket path. Defaults to `/var/run/docker.sock`.
+- `DOCKER_HOST`: optional `tcp://host:port` Docker API endpoint.
+
+## Artifact
+
+Container image from `Dockerfile`.
+
+## Notes
+
+Dockge does not currently expose a stable documented plugin API. This repository is structured as a read-only companion service that can run beside Dockge and inspect the same stack directory plus Docker Engine metadata.