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.yml

@@ -1,6 +1,7 @@
 services:
   bodenfeuchte:
     build: .
+    image: docker.citysensor.de/bodenfeuchte:latest
     expose:
       - "3000"
     volumes: