image: always use mount units for bootc disk images

See osbuild/bootc-image-builder#756

pkg/image/bootc_disk.go

@@ -68,6 +68,7 @@ func (img *BootcDiskImage) InstantiateManifestFromContainers(m *manifest.Manifes
 	rawImage.Directories = img.Directories
 	rawImage.KernelOptionsAppend = img.KernelOptionsAppend
 	rawImage.SELinux = img.SELinux
+	rawImage.MountUnits = true // always use mount units for bootc disk images
 
 	// In BIB, we export multiple images from the same pipeline so we use the
 	// filename as the basename for each export and set the extensions based on