You're right that Docker is similar to Nix, at least in the sense they both seem to be trying to work around problems with the Linux packaging and library ecosystem by piling more of their own complexity on top. I suspect the comment you replied to wants to see the actual underlying problem solved.

To use an example from another community, no amount of performance improvements to NPM will ever make it a good idea to depend on hundreds of one-liner "is number odd" or "left pad" packages. Papering over the problem with yet more technology only ossifies it, making it harder to solve for real.