doc.nix

{ config, lib, pkgs, ... }:

with lib;

let
  cfg = config.profiles.doc;
  homeDir = config.home.homeDirectory;
in
{
  options = {
    profiles.doc = {
      enable = mkEnableOption "doc";
      srcPath = mkOption {
        # default = homeDir + "/bootstrap/docs"; # <- why is this failing to do string context ?
        default = /home/vagrant/boostrap/docs; # works
        type = types.path;
      };
    };
  };
  config = mkIf cfg.enable {
    home.file.".local/share/devbox".source =
      let
        indexFile = "${cfg.srcPath}/modules/ROOT/pages/index.adoc";
      in
      pkgs.runCommand "doc"
        {
          preferLocalBuild = true;
          allowSubstitutes = false;
          buildInputs = [ pkgs.asciidoctor ];
        }
        ''
          mkdir $out
          cp -r ${cfg.srcPath}/modules/ROOT/assets/images $out
          asciidoctor ${indexFile} -a docinfodir=${cfg.srcPath} -a imagesdir=$out/images -o $out/devbox.html
          asciidoctor ${indexFile} -r asciidoctor-pdf -b pdf -a imagesdir=${cfg.srcPath}/modules/ROOT/assets/images -o $out/devbox.pdf
        '';
  };
}

I need to use ./, the root path literal:

srcPath = /. + config.home.homeDirectory + /bootstrap/docs;