Add a guide for Dockerfile

ref: https://github.com/moby/buildkit/blob/master/docs/build-repro.md

Applying the reproducible builds to the Docker Official Images (DOI) on
Docker Hub is being discussed in: https://github.com/docker-library/official-images/issues/16044


(waiting for the PRs linked there to be reviewed)

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>