Docker just hides the dirt under the carpet. It doesn’t actually clean up the mess. The beauty of something like NixOS or Guix is that you get reproducibility, immutability and hackability without any kind of container magic and you get it at the level of the individual package. The whole system is little more than a naming convention, some symlinks and environment variables. It’s basically all classic Unix’y stuff.
That said, other immutable solution that aren’t NixOS or Guix aren’t that much different from Docker from what I understand.