Looks like you gave me two working solutions!
I knew about .dockerignore, but after your message I realized I could potentially build dockerignore on fly, which would contain just two lines - ignore everything from bin/ except the binary I need, something like
bin/*
!bin/TheBinaryINeedForThisImage.zip
I also read that link earlier and already tested DOCKER_BUILDKIT=1, but didn’t test it correctly, because I saw it still transfer the context on early stage, but it actually analyzes the docker file and transfers only what is used in COPY commands. So, this is a working solution, exactly what I need.
Thank you bro, great job!