fix(backend): Possibility to use `SERVER_URL` to overwrite upload url (e.g. proxy http -> https)

[Ninos]

With SERVER_URL as environment variable it's possible now to change the image urls to "hardcoded" url. If not available, old method is used.

Currently this is important, if you're using a proxy between, because otherwise req.protocol is always http -> public image urls has http-scheme instead of https.