Build History for branch/buildkit

Use BuildKit features in Dockerfile - cache apt packages; - cache opam packages; - use COPY --link for faster copy; - sort package lists. https://docs.docker.com/reference/dockerfile/#example-cache-apt-packages https://docs.docker.com/reference/dockerfile/#benefits-of-using---link https://docs.docker.com/build/building/best-practices/#sort-multi-line-arguments