Add multi-platform build script and prod compose for registry deployment

- build-push.sh: buildx pipeline to docker.citysensor.de
- docker-compose.prod.yml: pull-only compose for nuccy (no build context)
- docker-compose.yml: image tag added for local tagging

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

docker-compose.prod.yml

@@ -0,0 +1,23 @@
+services:
+  bodenfeuchte:
+    image: docker.citysensor.de/bodenfeuchte:latest
+    expose:
+      - "3000"
+    volumes:
+      - bodenfeuchte-data:/app/data
+    env_file:
+      - .env
+    restart: unless-stopped
+
+  nginx:
+    image: nginx:alpine
+    ports:
+      - "80:80"
+    volumes:
+      - ./nginx/default.conf:/etc/nginx/conf.d/default.conf:ro
+    depends_on:
+      - bodenfeuchte
+    restart: unless-stopped
+
+volumes:
+  bodenfeuchte-data: