* Faster builds: Nix is better at (safely) caching the results of intermediate steps in your build, whereas a Dockerfile has to be conservative (if you so much as change the whitespace in a RUN command, that step (and all subsequent steps) have to be re-run).
If you've already bought into Docker, you can actually use Nix to construct your Docker images: https://nixos.org/nixpkgs/manual/#sec-pkgs-dockerTools
Some benefits:
* Nix provides reproducible docker images; Dockerfiles don't.
* Faster builds: Nix is better at (safely) caching the results of intermediate steps in your build, whereas a Dockerfile has to be conservative (if you so much as change the whitespace in a RUN command, that step (and all subsequent steps) have to be re-run).