Cut image size to 356 MB, down from 2 GB

Using a Docker feature called "multi-stage builds" [1], we build Maiko in
one image, then create a new one into which we copy the final binary but
not sources and headers. Both images are still Ubuntu-based (todo), but
the <400 MB final product is much slimmer than what we had before.

[1]: https://docs.docker.com/develop/develop-images/multistage-build/

Dockerfile

@@ -1,16 +1,24 @@
 FROM ubuntu:focal
 ENV DEBIAN_FRONTEND=noninteractive
 
-RUN apt-get update
-RUN apt-get install -y build-essential clang libx11-dev tightvncserver xfce4 xfce4-goodies
-COPY . /build
-WORKDIR /build/maiko/bin
+RUN apt-get update && apt-get install -y build-essential clang libx11-dev
+COPY maiko /build/
+WORKDIR /build/bin
+RUN rm -rf /build/linux*
 RUN ./makeright x
+
+
+FROM ubuntu:focal
+ENV DEBIAN_FRONTEND=noninteractive
+
+EXPOSE 5900
+
+RUN apt-get update && apt-get install -y tightvncserver
 RUN mkdir /app
-RUN cp ../linux.x86_64/* /app
-COPY loadups/xfull35.sysout /app
+WORKDIR /app
+COPY loadups ./
+COPY --from=0 /build/linux.x86_64/* ./
 
 RUN adduser --disabled-password --gecos "" medley
 USER medley
-WORKDIR /app
 ENTRYPOINT USER=medley Xvnc -geometry 1270x720 :0 & DISPLAY=:0 /app/ldex -g 1280x720 xfull35.sysout