The guy posted cross posted it to c/OpenSource, so I guess

Sounds pretty scary to me. The thing has full control over your computer (if infected) and is nearly undetectable.

You can even have all nixpkgs on a different distro through the nix package manager

That is very cool. Thanks for sharing. If you need some additional repos (like OP), you can probably add them through some commands in the “pre_init_hooks” option.

It should be possible. Although probably very complicated. Have a look at https://distrobox.it/. It allows you to tightly integrate containers into your desktop, including accelerated graphics, some devices, your homedir, etc. It can even automatically install desktop shortcuts. (You can disable the integrations of course) Even tho it uses Podman instead of docker, AFAIK it should be 1:1 compatible with docker for your usecase.