Configure the value of ROOT_URL without the domain name

[pinghe]

I do usually install the app to a subdomain like https://app1.example.com behind a reverse proxy.

I want to configure the value of ROOT_URL to be the path to the same domain. For example.
ROOT_URL = / or ROOT_URL = /app1.

But this configuration will report an error on startup:
Error: Must pass options.rootUrl or set ROOT_URL in the server environment

[jamesgibson14]

This would be great, I run 5-10 apps per server/meteor instance via sub domains, but I can only build one mobile app per instance because it always passes the same ROOT_URL to the mobile build instead of whatever sub domain is accessing the server.