In many discussions about Nix, the comparison of Nix and Docker comes up frequently. This question could be dismissed by saying that Nix and Docker are different tools that solve different problems. One is a toolkit for building and deploying containers and the other is a package and configuration manager. However, these tools do have some overlap: they can both be used to create reproducible environments. A reproducible environment is one that can be recreated from scratch in an identical way (ideally bit-for-bit). Practically, this means having the same tools, versions, and configuration between the environments.
Source: Will Nix Overtake Docker?, an article by Connor Brewster.