This prevents weird issues where a commit can happen between when the i386 binaries are built and when amd64 binaries are built, and also speeds up the workflow a bit.
Example usage, to build i386 libs: docker build -t sdl2-i386 -f Dockerfile.i386 . mkdir i386 docker run -v `pwd`/compile:/entry -v `pwd`/i386:/io sdl2-i386 s/i386/amd64 to build for amd64.