I'm not really sure what your point is, but I'll try to take it in good faith and read it as "why doesn't docker solve the problem for it, since you can also keep those configurations in a git repo?"

If any kind of apt upgrade or similar command is run in a dockerfile, it is no longer reproducible. Because of this it's necessary to keep track of which dockerfiles do that and keep track of when a build was performed; that's more out-of-band logging. With NixOS I will get the exact same system configuration if I build the same commit (barring some very exotic edge cases)

Besides that, docker still needs to run on a system, which must also be maintained, so Docker only partly addresses a subset of the issue

If Docker works for you and you're not facing any issues with such a setup, then that's great. NixOS is the best solution for me