For the use case of "I need a Docker image with some programs", this command seems pretty neat. Nixery is a similar with Nix, where an image can be constructed ad-hoc by specifying the dependencies of an image's path.

That's cool. And I can imagine cases where it's useful.

Stuff like Nixery isn't itself enough to claim "nix is better than Docker", though. -- The questions I'd want to think about are "what happens when I run into a problem? how hard is it to fix this?". I feel like Nix does solve some problems that I've had with Dockerfiles. But there are also problems I've run into with Nix that I wouldn't've had with Nix.

I like nix well enough. But I've also used it enough that I'm not surprised if people have difficulties with it, and are more comfortable with other solutions.