Documentation: Docker examples could use a GUI one

[l0b0]

Problem

I'm trying to write a simple .nix file to create a container image which runs a GUI application, ideally as a non-root user. There's various snippets and hints around, but no solid, simple example like the ones in this repo.

Proposal

Create a Docker example which runs a small GUI application as a proof of concept.

Checklist

• [x] checked latest Nixpkgs manual (source) and latest NixOS manual (source)
• [x] checked open documentation issues for possible duplicates
• [x] checked open documentation pull requests for possible solutions

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[eclairevoyant]

I don't think this is something that fits well in docs, it's a specific usecase. Wiki would be a better fit.