fix(mpc-system): 添加 Go 代理配置解决网络问题

在所有 Dockerfile 中添加 GOPROXY 配置:
- 使用 goproxy.cn 和 goproxy.io 作为国内代理
- 使用 sum.golang.google.cn 作为 GOSUMDB
- 支持通过 build args 覆盖代理设置

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/mpc-system/services/account/Dockerfile

@@ -3,6 +3,11 @@ FROM golang:1.21-alpine AS builder
 
 RUN apk add --no-cache git ca-certificates
 
+# Set Go proxy for China
+ARG GOPROXY=https://goproxy.cn,https://goproxy.io,direct
+ENV GOPROXY=${GOPROXY}
+ENV GOSUMDB=sum.golang.google.cn
+
 WORKDIR /app
 
 COPY go.mod go.sum ./

backend/mpc-system/services/message-router/Dockerfile

@@ -3,6 +3,11 @@ FROM golang:1.21-alpine AS builder
 
 RUN apk add --no-cache git ca-certificates
 
+# Set Go proxy for China
+ARG GOPROXY=https://goproxy.cn,https://goproxy.io,direct
+ENV GOPROXY=${GOPROXY}
+ENV GOSUMDB=sum.golang.google.cn
+
 WORKDIR /app
 
 COPY go.mod go.sum ./

backend/mpc-system/services/server-party/Dockerfile

@@ -3,6 +3,11 @@ FROM golang:1.21-alpine AS builder
 
 RUN apk add --no-cache git ca-certificates
 
+# Set Go proxy for China
+ARG GOPROXY=https://goproxy.cn,https://goproxy.io,direct
+ENV GOPROXY=${GOPROXY}
+ENV GOSUMDB=sum.golang.google.cn
+
 WORKDIR /app
 
 COPY go.mod go.sum ./

backend/mpc-system/services/session-coordinator/Dockerfile

@@ -4,6 +4,11 @@ FROM golang:1.21-alpine AS builder
 # Install dependencies
 RUN apk add --no-cache git ca-certificates
 
+# Set Go proxy for China (use GOPROXY env from build args if provided)
+ARG GOPROXY=https://goproxy.cn,https://goproxy.io,direct
+ENV GOPROXY=${GOPROXY}
+ENV GOSUMDB=sum.golang.google.cn
+
 # Set working directory
 WORKDIR /app