Uses multi-stage builds to create images

follows along here loosely:

https://docs.docker.com/develop/develop-images/multistage-build/
https://blog.alexellis.io/mutli-stage-docker-builds/

Dockerfile

@@ -1,8 +1,13 @@
+FROM golang:1.10.1 as build
+WORKDIR /go/src/mcquay.me/lim
+COPY . .
+RUN CGO_ENABLED=0 GOOS=linux go build -o /usr/local/bin/lim .
+
 FROM alpine:latest
 RUN apk --no-cache add ca-certificates
 
 WORKDIR /
 
-COPY build/lim /bin/
+COPY --from=build /usr/local/bin/lim /bin/
 
 CMD ["/bin/sh", "-c", "cat /dev/urandom | /bin/lim 16b | xxd"]