docker: add docu references & remove duplicate code

[h0nIg]

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.

[basbebe]

nixConf is now unused.
It was a handy way to edit nix.conf for custom images.

How is this supposed to work now?

[h0nIg]

@basbebe shit, during the last merge commit i may have missed this, #13435

[basbebe]

That was quick, thanks!