ENV variables to overwrite config values

[MorrisJobke]

See nextcloud/server#3966

[schiessle]

@DaphneMuller this is maybe something for the new portal, to get it finally documented. Do you want to set it on the list?

[DaphneMuller]

@DaphneMuller this is maybe something for the new portal, to get it finally documented. Do you want to set it on the list?

added it to the list in https://github.com/nextcloud-gmbh/NextcloudPortal-New/issues/3 at "new articles to write"

[kesselb]

The current implementation only works for a subset of configuration objects.

#9143 (comment) for more details.