> It is not the same level of reproducibility like Nix and Bazel provides.

What is "the same level"?

> You can pin ruby to exact version you want with https://nixos.wiki/wiki/Overlays

That page says: "Overlays provide a method to extend and change nixpkgs"

I don't want to change something. I want to simply pin a version. This is neither simple, nor scalable, nor maintainable (also here: https://news.ycombinator.com/item?id=28591202):

  Overriding a version

  self: super:
  {
    sl = super.sl.overrideAttrs (old: {
      src = super.fetchFromGitHub {
        owner = "mtoyoda";
        repo = "sl";
        rev = "923e7d7ebc5c1f009755bdeb789ac25658ccce03";
        # If you don't know the hash, the first time, set:
        # sha256 = "0000000000000000000000000000000000000000000000000000";
        # then nix will fail the build with such an error message:
        # hash mismatch in fixed-output derivation '/nix/store/m1ga09c0z1a6n7rj8ky3s31dpgalsn0n-source':
        # wanted: sha256:0000000000000000000000000000000000000000000000000000
        # got:    sha256:173gxk0ymiw94glyjzjizp8bv8g72gwkjhacigd1an09jshdrjb4
        sha256 = "173gxk0ymiw94glyjzjizp8bv8g72gwkjhacigd1an09jshdrjb4";
      };
    });
  }