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>

build-push.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+REGISTRY="docker.citysensor.de"
+IMAGE="${REGISTRY}/bodenfeuchte"
+PLATFORMS="linux/amd64"
+TAG="${1:-latest}"
+
+# Ensure a buildx builder with multi-platform support exists
+if ! docker buildx inspect bodenfeuchte-builder &>/dev/null; then
+  docker buildx create --name bodenfeuchte-builder --use
+else
+  docker buildx use bodenfeuchte-builder
+fi
+
+echo "Building and pushing ${IMAGE}:${TAG} for ${PLATFORMS} ..."
+
+docker buildx build \
+  --platform "${PLATFORMS}" \
+  --tag "${IMAGE}:${TAG}" \
+  --push \
+  .
+
+echo "Done: ${IMAGE}:${TAG}"