Alternative domains support
As in https://docs.joinmastodon.org/admin/config/#basic :
ALTERNATE_DOMAINS
If you have multiple domains pointed at your Mastodon server, this setting will allow Mastodon to recognize itself when users are addressed using those other domains. Separate the domains by commas, e.g. foo.com,bar.com
Current situation is we can run another frontend at another domain (e.g. Soapbox) by purely calling the API at that domain, but the main domain is still required for things like media proxy.