But in order to build with BuildKit you MUST BE connected to the internet during the build... So not ready to go yet...
It was a deep hack into the entirety of the Docker build chain. This way it was probably possible to publish experimental work-in-progress build features to the world at a faster pace than the official Docker release cycle.
And as pointed out above, the online-only requirement has been lifted already.
The situation is perfectly understandable, and to be commended that people offered to do this work for the betterment of all.