docs: update installation guide to use pre-built image

Replace the clone-and-build workflow with a docker-compose snippet
that pulls the image directly from the Gitea container registry.
Add semver tag reference and rename steps to match the new flow.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -27,15 +27,36 @@ https://your-host/graph.svg?theme=dark
 
 - Docker + Docker Compose
 
-### 1. Clone and configure
+### 1. Create a `docker-compose.yml`
 
-```bash
-git clone https://git.arthurerlich.de/haylan/git-contribution-graph.git
-cd git-contribution-graph
-cp .env .env.local
+Use the pre-built image — no need to clone the repo:
+
+```yaml
+services:
+  graph:
+    image: git.arthurerlich.de/haylan/git-contribution-graph:latest
+    ports:
+      - "8080:8080"
+    env_file:
+      - .env.local
+    volumes:
+      - cache:/var/www/html/var/cache
+
+volumes:
+  cache:
 ```
 
-Edit `.env.local` with your credentials:
+The image is published to the Gitea container registry. Pull it manually with:
+
+```bash
+docker pull git.arthurerlich.de/haylan/git-contribution-graph:latest
+```
+
+Semver tags are published (`0`, `0.0`, `0.0.1`) alongside `latest` — see the [container registry](https://git.arthurerlich.de/haylan/-/packages/container/git-contribution-graph/latest) for all available tags.
+
+### 2. Configure
+
+Create a `.env.local` file next to your `docker-compose.yml`:
 
 ```dotenv
 APP_SECRET=<generate with: openssl rand -hex 16>
@@ -60,7 +81,7 @@ ALLOWED_HOSTS=
 
 Only configure the platforms you use — unused ones are silently skipped.
 
-### 2. Start
+### 3. Start
 
 ```bash
 docker compose up -d
@@ -68,10 +89,11 @@ docker compose up -d
 
 The service listens on **port 8080** by default. Put Traefik or nginx in front of it for HTTPS.
 
-### 3. Health check
+### 4. Verify
 
-```
-GET /health  →  {"status":"ok"}
+```bash
+curl http://localhost:8080/health
+# {"status":"ok"}
 ```
 
 ---