What is automatic about docker? Do you mean other people have already put in the work? Or do you mean that it's more trivial to pip/npm/cargo install stuff?

Nix as package management, yes - you're waiting on someone else to make upstream work.

Where Debian/PIP/NPM/Cargo already do work.

Nix as an instruction set, no, you still have to declare what you want, same as a dockerfile.

it's just... Docker will do what a shell script will.

Nix is like hoping someone's done the upstream finagling to get your particular dependencies happy.

If something works in a distro, it automatically works in a Dockerfile based on that distro. There is no extra packaging effort required.