Reproducible builds

Please add an option to allow reproducible builds.

Buildkit should provide most building blocks: https://github.com/moby/buildkit/blob/master/docs/build-repro.md

Of course the base `Dockerfile` author is responsible to make sure it allows reproducibility like installing fixed versions via their package manager (e.g. with [repro-get](https://github.com/reproducible-containers/repro-get)).