As written previously this seemed to suggest using "app.terraform.io" (the
"hostname you use to access the Terraform Cloud application) to access a
private registry in Terraform Enterprise, but that isn't true and I assume
isn't what was intended.
Instead, the hostname for a Terraform Enterprise instance is the hostname
where the Terraform Enterprise application is running, which is both the
hostname where users would find its web UI and the hostname they'd use
to configure the "remote" backend for remote operations and state storage.