Update Dockerfile generating smaller image

Dockerfile

@@ -1,14 +1,23 @@
-FROM golang:1.12-alpine as build
-LABEL maintainer="Andrea Spacca <andrea.spacca@gmail.com>"
+# Default to Go 1.12
+ARG GO_VERSION=1.12
+FROM golang:${GO_VERSION}-alpine as build
+
+# Necessary to run 'go get' and to compile the linked binary
+RUN apk add git musl-dev
 
-# Copy the local package files to the container's workspace.
 ADD . /go/src/github.com/dutchcoders/transfer.sh
 
+WORKDIR /go/src/github.com/dutchcoders/transfer.sh
+
+ENV GO111MODULE=on
+
 # build & install server
-RUN go build -o /go/bin/transfersh github.com/dutchcoders/transfer.sh
+RUN go get -u ./... && CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -ldflags -a -tags netgo -ldflags '-w -extldflags "-static"' -o /go/bin/transfersh github.com/dutchcoders/transfer.sh
+
+FROM scratch AS final
+LABEL maintainer="Andrea Spacca <andrea.spacca@gmail.com>"
 
-FROM golang:1.11-alpine
-COPY --from=build /go/bin/transfersh /go/bin/transfersh
+COPY --from=build  /go/bin/transfersh /go/bin/transfersh
 
 ENTRYPOINT ["/go/bin/transfersh", "--listener", ":8080"]