Added Dockerfile

Dockerfile

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+FROM golang:1.8.5-alpine as builder
+ARG GOPROXY_VERSION=master
+RUN apk update && apk upgrade && \
+    apk add --no-cache git && cd /go/src/ && git clone https://github.com/snail007/goproxy && \
+	cd goproxy && git checkout ${GOPROXY_VERSION} && \
+    go get && CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o proxy
+FROM alpine:3.7
+COPY --from=builder /go/src/goproxy/proxy /
+CMD /proxy ${OPTS}