docs: Mention Format=none in sysext doc

This allows disabling building of the default image as requested
in #3113.

diff --git a/docs/sysext.md b/docs/sysext.md
index edfe48e95326fc99655e6b012940155fc69eff4c..b82e162db56645027a0c4683b69149dc80bda94d 100644 (file)
--- a/docs/sysext.md
+++ b/docs/sysext.md
@@ -93,3 +93,5 @@ BaseTrees=%O/base
 
 If we now run `mkosi -f` again, we'll find `initrd.cpio.zst` in
 `mkosi.output` with its accompanying extension still in `btrfs.raw`.
+If you don't have any need for a main image, you can configure
+`Format=none` in the `Output` section in `mkosi.conf` to disable it.