What is the documentation issue?

VERCEL_URL is supposed to be available at build time but isn't which breaks local deployment workflows.

Is there any context that might help us understand?

https://community.vercel.com/t/vercel-url-not-available-at-build-time-if-built-outside-of-vercel/16568

Does the docs page already exist? Please link to it.

https://vercel.com/docs/environment-variables/system-environment-variables#VERCEL_URL