Support building apps from nix flakes

[CourteousCoder]

Feature request

This could be implemented by including the project's flake as the first of the nix dependencies when building the Dockerfile

This talk (thanks, @Xe 👍) is a great resource for implementation.

The nix-flakes provider would be detected if there is a flake.nix and the flake passes a nix flake check.

Motivation

This feature would solve building from providers that are not yet supported by nixpacks as long as the flake can be built purely from a nix flake.

Contribution

I'd be happy to take a stab at writing a PR for this one!