One difference is that Docker containers use a separate file system isolated from the host, so you have to separately install your editor/shell in there, mount/clone your dotfiles, etc. With a Nix-based development environment, it can add/override the tools you need, but you get to keep your shell customizations, editor config, etc.

Also reproducibility; it can be achieved with containers if you save the artifact (the container image), but that's not what people do in practice, they save only the recipe (the Dockerfile), and if you execute it tomorrow, it will produce a different result than today, and it will likely not even run one year from now (due to e.g. third-party apt repos changing their url, signing keys expiring, curl|bash installers that are no longer hosted, etc.). With Nix, every run will produce the same results, and sources for everything packaged in Nixpkgs are saved by nixos.org.