nodered: bake Dashboard 2.0 into Dockerfile for idempotent install

NR's automatic package.json install at container start was slow and
required internet. Move npm install to image build time, preserve the
installed node_modules via an anonymous volume so the bind mount of
./nodered:/data doesn't clobber them.

infra/nodered/package.json

@@ -0,0 +1,9 @@
+{
+  "name": "node-red-project",
+  "description": "Node-RED flows for Klubhaus zigbee-monitor",
+  "version": "0.0.1",
+  "private": true,
+  "dependencies": {
+    "@flowfuse/node-red-dashboard": "1.30.2"
+  }
+}