build(webdav): 添加GOPROXY环境变量以加速构建

在构建阶段添加GOPROXY环境变量，使用国内镜像源加速依赖下载

webout/webdav/dockerfile

@@ -1,6 +1,8 @@
 # RUN: docker buildx build --push --platform linux/amd64,linux/arm64 -t cnphpbb/caddy:2-webdav-alpine .
 FROM caddy:2.10-builder-alpine AS builder
 
+ENV GOPROXY=https://goproxy.cn,direct
+
 RUN xcaddy build --with github.com/mholt/caddy-webdav
 
 FROM caddy:2.10-alpine