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>

frontend/Dockerfile

@@ -1,5 +1,6 @@
 # Build stage
-FROM node:20-alpine AS builder
+ARG DOCKER_MIRROR=""
+FROM ${DOCKER_MIRROR}node:20-alpine AS builder
 
 # Use Aliyun mirrors for Alpine packages
 RUN sed -i 's/dl-cdn.alpinelinux.org/mirrors.aliyun.com/g' /etc/apk/repositories
@@ -29,7 +30,7 @@ RUN mkdir -p public
 RUN npm run build
 
 # Production stage
-FROM node:20-alpine AS runner
+FROM ${DOCKER_MIRROR}node:20-alpine AS runner
 
 # Use Aliyun mirrors for Alpine packages
 RUN sed -i 's/dl-cdn.alpinelinux.org/mirrors.aliyun.com/g' /etc/apk/repositories