is there still relevance for this way of building in the world of Docker - especially with Packer and LinuxKit ?
Packer allows us to create highly repeatable OS builds with all the right configuration and packaging. For example, this is AWS official Packer build for their EKS AMIs - https://github.com/awslabs/amazon-eks-ami
Docker kind of sucks for development though, because you need to reinstall all your devtools into the container and have to wire up your IDE to support docker. With nix, you just write a shell.nix, run nix-shell and can then use your regular tools in that environment.
Also, nix builds are far more reproducible than packer builds. With nix, you get an environment that is guaranteed to be identical down to the hash of every single file as long as you use the same version of nixpkgs. Packer and similar imperative tools typically install whatever version is the latest in the distro repo, and building the image 2 months apart will grant you different artifacts without investing a lot of effort into pinning everything.
Packer allows us to create highly repeatable OS builds with all the right configuration and packaging. For example, this is AWS official Packer build for their EKS AMIs - https://github.com/awslabs/amazon-eks-ami