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/compose.yaml

@@ -39,7 +39,7 @@ services:
   #     TZ: America/Los_Angeles
   #
   nodered:
-    image: nodered/node-red:latest
+    build: ./nodered
     container_name: nodered
     restart: unless-stopped
     depends_on:
@@ -48,11 +48,12 @@ services:
     ports:
       - "1880:1880"
     volumes:
-      - ./nodered:/data
+      - ./nodered:/data                       # bind: settings.js, flows.json, start.sh, package.json
+      - /data/node_modules                    # anonymous: preserves build-time npm install
     environment:
       TZ: America/Los_Angeles
       DASHBOARD_BASE_HREF: /dashboard/
-    command: ["sh", "/data/start.sh"]
+    command: ["sh", "/data/start.sh"]	
 
   # Listens to ntfy.sh for doorbell events and toggles a smart plug on each alert.
   # Topic: ALERT_klubhaus_topic_test (matches ESP32 firmware DEBUG_MODE suffix)