Add configurable Docker mirror for China network builds

Add DOCKER_MIRROR build arg to Dockerfiles so base images can be
pulled through a mirror registry (e.g. docker.1ms.run/) when Docker
Hub is unreachable. Defaults to empty (direct Docker Hub).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.deploy.yml

@@ -5,6 +5,8 @@ services:
     build:
       context: ./backend_rust
       dockerfile: Dockerfile
+      args:
+        DOCKER_MIRROR: ${DOCKER_MIRROR:-}
     container_name: notify-backend
     restart: unless-stopped
     extra_hosts:
@@ -23,6 +25,7 @@ services:
       context: ./frontend
       dockerfile: Dockerfile
       args:
+        DOCKER_MIRROR: ${DOCKER_MIRROR:-}
         NEXT_PUBLIC_API_BASE: ${NEXT_PUBLIC_API_BASE:-http://localhost}
     container_name: notify-frontend
     restart: unless-stopped