dockerfile: Use multi-stage build for much smaller sizes

Dockerfile

-FROM golang:rc
-
-EXPOSE 6667/tcp 6697/tcp
+# build Oragono
+FROM golang:rc AS build-env
 
 RUN apt-get install -y git
 
 RUN git clone https://github.com/oragono/oragono.git
 WORKDIR /go/src/github.com/oragono/oragono
 RUN git submodule update --init
-RUN go build oragono.go
+RUN make linux
+
+# run in Alpine, being a lightweight distro
+FROM alpine:latest
+EXPOSE 6667/tcp 6697/tcp
+
+RUN mkdir -p /ircd
+WORKDIR /ircd
 
-COPY run.sh /go/src/github.com/oragono/oragono
+COPY --from=build-env /go/src/github.com/oragono/oragono/build/oragono-XXX-linux.tgz /
+RUN tar -xzf /oragono-XXX-linux.tgz
+COPY run.sh /ircd
 
 CMD ["./run.sh"]