extra-
From the nix manual: A configuration setting usually overrides any previous value. However, for settings that take a list of items, you can prefix the name of the setting by extra- to append to the previous value.
Include usage from NixOS and non-NixOS hosts and a test with curl to ensure it can be reached. Reviewed-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>