Add Docker deploy for Caddy production at cyd.smallzhiyuns.com.

Standalone Next image, local firmware sync volume, and docs for the existing host-network Caddy reverse proxy.

Co-authored-by: Cursor <cursoragent@cursor.com>

docker-compose.yml

@@ -0,0 +1,18 @@
+services:
+  cydweb:
+    build:
+      context: .
+      args:
+        NEXT_PUBLIC_SITE_URL: https://cyd.smallzhiyuns.com
+    image: cydweb:latest
+    container_name: cydweb
+    restart: unless-stopped
+    ports:
+      - "127.0.0.1:3020:3000"
+    environment:
+      - NODE_ENV=production
+      - NEXT_PUBLIC_SITE_URL=https://cyd.smallzhiyuns.com
+      - CYD_FIRMWARE_ROOT=/app/firmware
+    volumes:
+      # Live-update firmwares without rebuild (host path populated by sync)
+      - ./firmware:/app/firmware:ro