Docker container build in tutorial

[utsl42]

I think it would be useful to show an actual flake example of building a Docker container in the tutorial, or in follow on documentation. This answers the question of, "So I built a development environment. Now what?".

Currently https://zero-to-nix.com/start/learn-more takes you straight to https://github.com/NixOS/nixpkgs/blob/master/pkgs/build-support/docker/examples.nix, which is a lot to drop someone who just finished that tutorial into.

Even changing the link to https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-bundle.html would be an improvement, since that would be showing something much more approachable for a beginner than a raw .nix file, which at that point in learning Nix, they can't be reasonably expected to understand.