修复构建镜像问题

backend/Dockerfile

@@ -6,6 +6,7 @@
 #   docker build -f backend/Dockerfile \
 #     --build-arg GOLANG_IMAGE=<working-mirror>/golang:1.21-alpine \
 #     --build-arg RUNTIME_IMAGE=<working-mirror>/alpine:3.20 \
+#     --build-arg GOPROXY=https://<internal-go-proxy>,direct \
 #     -t REGISTRY/PROJECT/zhinian-go-api:TAG backend/
 #
 # If no mirror can serve the golang image at all, use backend/Dockerfile.runtime
@@ -15,7 +16,12 @@ ARG RUNTIME_IMAGE=alpine:3.20
 
 FROM ${GOLANG_IMAGE} AS build
 WORKDIR /src
-ENV CGO_ENABLED=0 GOOS=linux
+ARG GOPROXY=https://goproxy.cn,direct
+ARG GOSUMDB=sum.golang.google.cn
+ENV CGO_ENABLED=0 \
+    GOOS=linux \
+    GOPROXY=${GOPROXY} \
+    GOSUMDB=${GOSUMDB}
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .