Communicating via container URLs works, but the URL provided to
livekit-jwt-service as `LIVEKIT_URL` is also passed to the user later
and it must be a public one at that point.

It'd be great if livekit-jwt-service can be given 2 different URLs
(e.g. `LIVEKIT_URL` and `LIVEKIT_URL_PUBLIC`) and only announce the
public one to the user, but there's no support for this yet.