While I understand the argument you're making, the exposure of running "curl | bash" on your local machine is much higher than running arbitrary code inside of a container.
Even if you specify a hash, are you actually checking all of the code and binaries in that image?
What about the image base (e.g. Alpine, Debian) and their packages?
I mention this because Flakes can do much of what Docker can do, but do not use containers or virtualization.
The official NixPkgs are carefully vetted. Nix Flakes make it more convenient to pull in sources from outside NixPkgs.
As Nix Flakes become more prevalent, perhaps there needs to be some way to containerize them. There are more benefits to Nix Flakes than native execution so containerizing them somehow might be an acceptable compromise.
You may be interested in nix snapshotter, which can run nix packages in containerd.