That doesn't satisfy the requirement "from source". Sure, you can build on a large binary blob that already contains everything you need, but then you are just moving the issue into the build process of that blob.

Also, 99% of container image build scripts (Dockerfiles, etc.) probably don't pin an exact base image and don't avoid pulling in external resources (e.g. packages from the base image distro's repositories).

I wouldn't call a system that makes it very easy to break reproducible build environments easier than just using nix to achieve that reproducibility.