I guess you could consider a docker image a "reproducible environment," but it's certainly not a reproducible build; running docker build twice on the same directory isn't guaranteed to give you the same image. You could put in the work to make it a reproducible build, but it doesn't do anything to help you achieve that. Nix defaults to reproducible builds, and requires flags for "impure" non-reproducible builds. It does this by requiring all dependencies be managed by nix, and all sources be copied into the nix store.