No offense but it sounds like you don’t actually understand nix flakes if you think they’re 1:1 equivalent to Docker.
They simply are not containers. They allow the declarative BUILD of any derivation at any time in the future. They hermetically lock all dependencies and build instructions which allows you to archive and reproduce the EXACT content-addressed dependency graph of the software. You can rebuild using a flake while Docker doesn’t actually allow that same hermetic reproducible guarantee whatsoever.