feat: 更新 Dockerfile，添加 GOPROXY 环境变量

backend/Dockerfile

@@ -1,6 +1,7 @@
 FROM golang:1.26-alpine AS builder
 WORKDIR /src
 COPY go.mod go.sum ./
+ENV GOPROXY=https://goproxy.cn
 RUN go mod download
 COPY . .
 RUN CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o /out/api ./cmd/server