Add a standalone docker-compose.yml example using the pre-built image

Complements the existing "build: ." example (for a full checkout) with
a minimal compose file that just pulls git.arnef.de/arnef/nidus:latest,
for users who only want to run the container without cloning the repo.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

README.md

@@ -110,14 +110,30 @@ Pushing a version tag (e.g. `v1.2.3`) or publishing a release triggers
 which builds and publishes a multi-arch (`linux/amd64` + `linux/arm64`)
 image to `git.arnef.de/arnef/nidus`, tagged with the version, `<major>.<minor>`,
 `latest`, and the short commit SHA. It authenticates via the
-`REGISTRY_USERNAME`/`REGISTRY_PASSWORD` repository secrets. Point
+`REGISTRY_USERNAME`/`REGISTRY_PASSWORD` repository secrets.
-`docker-compose.yaml`'s `image:` at it instead of `build: .` to use it
+
-directly, e.g.:
+A minimal standalone `docker-compose.yml` that pulls this image instead of
+building from a checkout — just fetch `config.example.yaml`, copy it to
+`config.yaml`, and adjust it to your needs:
 
 ```yaml
 services:
   davserver:
     image: git.arnef.de/arnef/nidus:latest
+    ports:
+      - "8080:8080"
+    volumes:
+      - ./config.yaml:/app/config.yaml:ro
+      - dav-data:/app/data
+    restart: unless-stopped
+    healthcheck:
+      test: ["CMD", "wget", "-qO-", "http://localhost:8080/healthz"]
+      interval: 30s
+      timeout: 5s
+      retries: 3
+
+volumes:
+  dav-data:
 ```
 
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