Speedup build_in_docker by only copying the necessary files

Author: eregon

Rakefile

@@ -83,7 +83,7 @@ task :build_in_docker => :templates do
   versions.each do |version|
     dockerfile = <<~DOCKERFILE
     FROM docker.io/library/gcc:#{version}
-    ADD . /prism
+    COPY --parents Makefile src include /prism/
     WORKDIR /prism
     RUN gcc --version
     RUN make SOEXT=so