GITEA_HOSTNAME was doing double duty: a bare hostname for the
start_pre() MagicDNS getent check, and (implicitly, via manual
instructions) a base for the registration URL. Since Gitea moved to
`tailscale serve --https=443` (#19), the instance is reachable on 443
with no port — the old implicit `http://<hostname>:3000` no longer
applies.
GITEA_HOSTNAME stays a bare hostname; GITEA_INSTANCE_URL is a new,
independently overridable variable defaulting to
https://${GITEA_HOSTNAME}, forwarded through exec_in_lxc and used in
the printed registration command.
The instance URL is frozen into .runner at registration time, so
changing the variable doesn't retroactively fix an already-registered
runner. check_runner_url_drift() detects a mismatch and warns with
the re-registration procedure, without touching .runner — deleting a
working runner's registration is a deliberate, manual, documented
step, not something a rerunnable install script should do on its own.
Closes#22
Damien