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>

backend_rust/Dockerfile

@@ -1,5 +1,6 @@
 # Build stage
-FROM rust:1.85-alpine AS builder
+ARG DOCKER_MIRROR=""
+FROM ${DOCKER_MIRROR}rust:1.85-alpine AS builder
 
 # Use Aliyun mirrors for faster package downloads in China
 RUN sed -i 's/dl-cdn.alpinelinux.org/mirrors.aliyun.com/g' /etc/apk/repositories
@@ -40,7 +41,7 @@ RUN touch src/main.rs migration/src/main.rs migration/src/lib.rs
 RUN cargo build --release
 
 # Runtime stage
-FROM alpine:3.21
+FROM ${DOCKER_MIRROR}alpine:3.21
 
 # Use Aliyun mirrors
 RUN sed -i 's/dl-cdn.alpinelinux.org/mirrors.aliyun.com/g' /etc/apk/repositories