Allow setting base url for reverse proxy support

[pannal]

It'd be great to have reverse proxy support, such as subdomains and/or subfolders.

[JonBunator]

I haven't tried it yet. But shouldn't this already work with nginx or traefik?

[pannal]

Subdomains work, but prefixes/subfolders don't as assets are referenced via ./, which results in https://domain.uri/assets not https://domain.uri/folder/assets

[JonBunator]

Ah okay, you want to set the base url. This should be easy to implement via an environment variable called BASE_URL.
@SavageCore already implemented it on his fork: SavageCore@54269e1
If someone wants to do a PR, this should be fairly easy to add.