fix: move GOPROXY setup before go mod download with fallback sources

Dockerfile

@@ -2,17 +2,17 @@ FROM golang:1.26-alpine AS builder
 
 WORKDIR /app
 
-# 设置 Go 国内镜像源
-RUN go env -w GOPROXY=https://goproxy.cn,direct && \
-    go env -w GOSUMDB=off
-
 # 设置 Alpine 国内镜像源
 RUN sed -i 's/dl-cdn.alpinelinux.org/mirrors.tuna.tsinghua.edu.cn/g' /etc/apk/repositories
 
 RUN apk add --no-cache gcc musl-dev
 
 COPY go.mod go.sum ./
-RUN go mod download
+
+# 设置 Go 国内镜像源（多备用源）
+RUN go env -w GOPROXY=https://goproxy.cn,https://goproxy.io,direct && \
+    go env -w GOSUMDB=off && \
+    go mod download
 
 COPY . .