web UI on different domain than activitypub endpoints

[lillian-b]

Similar to #28503

I have a mydomain.com where I want to have accounts (@me@mydomain.com). I want the Mastodon frontend to be on a different domain (mastodon.vpn.mydomain.com).

• I first tried configuring this by setting the LOCAL_DOMAIN to mydomain.com and the WEB_DOMAIN to mastodon.vpn.mydomain.com, but endpoints this results in the `mastodon.vpn.mydomain.com domain being set in ActivityPub endpoints.
• I tried removing LOCAL_DOMAIN, and now it is correctly setting mydomain.com in ActivityPub endpoints, but I get 403 when trying to access the instance through mastodon.vpn.mydomain.com.

What is the correct way to do this?

[lillian-b]

I dug through the code and found ALTERNATE_DOMAINS environment variable, which works for this.