> Then you would have a system dependent on the volatile state you configured by hand, meaning the system configuration is not declarative or reproducible.

If you're using Ansible as the author already was, you essentially have this already. I can do a one command deploy to cloud servers, dedicated servers and colo boxes with just Ansible. Docker gets you a slightly more guaranteed environment and an additional layer of abstraction (which has its own set of pros and cons), but that's about it.