docker: add docu references & remove duplicate code (backport #13328)

[mergify]

Motivation

we build a docker image with nix.
docker.nix is complex + without docu references and docker.nix looks like a template rather than a reuse function. General functionality which do not need a refactoring should get moved to nixpkgs, in order to avoid code duplication.

Context

https://discourse.nixos.org/t/how-to-build-a-docker-image-with-a-working-nix-inside-it/32960

I looked at the source code and expected to find maybe 200 lines to bootstrap Nix but instead I found almost 800 lines and too many things happening for how seemingly simple the end goal is.

---

This is an automatic backport of pull request #13328 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of e72a0ad has failed:

On branch mergify/bp/2.28-maintenance/pr-13328
Your branch is up to date with 'origin/2.28-maintenance'.

You are currently cherry-picking commit e72a0ad8c.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   docker.nix

no changes added to commit (use "git add" and/or "git commit -a")

Cherry-pick of 8fbc27a has failed:

On branch mergify/bp/2.28-maintenance/pr-13328
Your branch is ahead of 'origin/2.28-maintenance' by 2 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 8fbc27af4.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   docker.nix

no changes added to commit (use "git add" and/or "git commit -a")

Cherry-pick of ba12adc has failed:

On branch mergify/bp/2.28-maintenance/pr-13328
Your branch is ahead of 'origin/2.28-maintenance' by 3 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit ba12adc0f.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   docker.nix

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally