Convert build.sh to a multi-stage Dockerfile. (#452)

environments/binary/Dockerfile

@@ -1,10 +1,18 @@
+FROM golang:onbuild
+
+WORKDIR /go
+COPY *.go /go/
+
+RUN GOOS=linux GOARCH=386 go build -o server .
+
 FROM alpine:3.5
 
+WORKDIR /app
+
 RUN apk update
 RUN apk add coreutils binutils findutils grep
 
-COPY . /app
-WORKDIR /app
+COPY --from=0 /go/server /app/server
 
 EXPOSE 8888
-ENTRYPOINT ["./server"]
+ENTRYPOINT ["./server"]