polishing of Dockerfile

build/Dockerfile

@@ -1,18 +1,19 @@
 ### Build stage
-FROM golang:alpine AS build-env
+FROM golang:alpine3.9 AS build-env
+ENV VERSION v2.0
 # Install build deps
 RUN apk update && apk --no-cache add git gcc build-base
 # Get the source code
-RUN git clone https://github.com/vipnode/vipnode.git
+RUN git clone -b ${VERSION} https://github.com/vipnode/vipnode.git
 
 WORKDIR /go/vipnode
 RUN go get -d -v
 RUN go mod download
 RUN GOOS=linux GOARCH=amd64 go build -ldflags="-w -s" -o vipnode
 
 ### Final stage
-FROM alpine
+FROM alpine:3.9
 RUN apk update && apk --no-cache add ca-certificates
 WORKDIR /app
-COPY --from=build-env /go/vipnode/vipnode /app/
-ENTRYPOINT ./vipnode host --rpc=http://my.ethchain.dnp.dappnode.eth:8545 -vv --payout=${PAYOUT_ADDRESS} --nodekey=/app/.ethchain/network/key
+COPY --from=build-env /go/vipnode/vipnode /usr/bin/
+ENTRYPOINT vipnode host --rpc=http://my.ethchain.dnp.dappnode.eth:8545 -vv --payout=${PAYOUT_ADDRESS} --nodekey=/app/.ethchain/network/key