[FEATURE] Add BASE_URL environment variable for easier reverse proxy

[alaintxu]

When I try to use a reverse proxy, I usually try to set a URL as https://example.com/stump. But as no base url is set, it tries to access assets from https://example.com/assets and not from https://example.com/stump/assets .

I would like to be able to set an environment variable (e.g. BASE_URL at docker compose file) so the app knows that base url is not the root of the domain.

[aaronleopold]

I think this is related to #561