improve container build

Dockerfile

@@ -1,8 +1,9 @@
 FROM docker.io/library/golang:1.23-alpine
-ARG  GOPROXY
+ARG  GOPROXY=direct
+ARG  ALPINE_MIRROR=https://mirrors.ustc.edu.cn
 WORKDIR /src
 COPY go.mod go.sum ./
-RUN  apk add git && env GOPROXY=${GOPROXY:-direct} go mod download
+RUN  sed -i "s,https://dl-cdn.alpinelinux.org,${ALPINE_MIRROR}," /etc/apk/repositories; apk add git && env GOPROXY=${GOPROXY:-direct} go mod download
 ADD . /src
 RUN  go build -o /cache-proxy ./cmd/proxy