If you decide to try https://devenv.sh, it has OCaml support built-in:

  $ cat devenv.nix
  { config, ...}: 
  let
    ocamlPackages = config.languages.ocaml.packages;
  in { 
    languages.ocaml.enable = true;

    packages = [ ocamlPackages.janeStreet.async ];
  }

  $ devenv shell
  ...

Recently also shipped 0.6 with container generation support: https://devenv.sh/containers/

Let me know if you give it a try :)