Description of changes

The goal is to make the logic written for dockerTools.makeNixShellImage reusable outside of that context.
We can't factor it out wholesale, so I'd like to do it piece by piece, so that it's easy to piece together solutions for other, slightly different use cases as well.

These (simple) refactors will support use cases such as

• POC: devShell interface #206728
  • Without a Nixpkgs-based implementation of the shell functionality, it's not a real solution
  • Experimentation happens in leaps, quality happens by making small steps in the right direction
• SImplify gradle: add setup hook #272380

More concretely, this already

• Adds tests for the stringValue function
• Adds documentation

This is very repeatable, so if you like the goal or enjoy this kind of thing:
help out if you can :)

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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