Dockerfile: only copy the build directory artifacts into the final release image

Use the docker multi-stage build functionality to perform the Retro68 build in a
temporary build image, and then copy the final build directory into a fresh
release image. This reduces the final release image size from 9.2GB to 2.6GB.

Dockerfile

@@ -1,6 +1,7 @@
 # vim:ft=dockerfile
 
-FROM ubuntu:20.04
+# Base image
+FROM ubuntu:20.04 AS base
 
 RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt-get install -y \
@@ -8,8 +9,15 @@ RUN apt-get update && \
         libboost-all-dev bison texinfo \
         ruby flex curl g++
 
+# Build image
+FROM base AS build
+
 ADD . /Retro68
 
-RUN mkdir /Retro68-build
+RUN mkdir /Retro68-build && \
+    bash -c "cd /Retro68-build && bash /Retro68/build-toolchain.bash"
 
-RUN bash -c "cd /Retro68-build && bash /Retro68/build-toolchain.bash --clean-after-build"
+# Release image
+FROM base AS release
+
+COPY --from=build /Retro68-build/toolchain /Retro68-build/toolchain