If you're looking for immutability, I can recommend NixOS. However what is drawing me to container Linux is mainly its update process
I'm using NixOS happily since 2014 after getting messed up in pacman dependencies on Arch and haven't looked back. But their too radical shift from LSB makes for a less attractive enterprise story. Hopefully GuixSD[1] can plug that hole due to the GNU brand as it has been progressing nicely.
We've also built https://github.com/vpsfreecz/vpsadminos recently which is based on not-os / NixOS