Insecure Registry Example is Incorrect for Buildkit

[da1910]

File: registry/insecure.md

With buildkit you must provide "http://" in your insecure registry URL, just providing the hostname or "https://" does not work. The documentation does not make this clear, and with new(ish) versions of Docker Desktop following the documentation does not help, no useful error is raised and it leads to confusion and annoyance.

See related issue with moby and buildkit: https://github.com/moby/moby/issues/41059