Nixery is a cool idea. The use case for something like Nixery is a more narrow "I want an image with these packages installed". Whereas, with Dockerfiles you've got more flexibility in the image you build.
Hmm not really. That's the toy use case. Nixery builds your expression and gives you a docker container. By default you can specify like "git and tmux" as they are the example toy expressions HOWEVER it becomes a different beast when you use it in monorepo situation see https://GitHub.com/ghuntley/ghuntley and https://code.tvl.fyi/
My understanding is that's still "use Nixery to build docker image with packages installed", but it's a different set of packages being served.
What use cases / dynamics arise from doing that? & what's the value-add from doing that with Nixery compared to just otherwise building docker images with dockerTools in Nixpkgs?