When is an apkovl file applied? When booting Alpine, one can specify an overlay file via the kernel command line argument `apkovl=`. This overlay file is fetched and expanded over the rootfs. What is responsible for doing this? When during boot does it happen?

The initramfs contains the script `/init` which is generated from `initramfs-init.in`.

The script first downloads the overlay file if it is on the network. Then it calls the function `unpack_apkovl` which optionally decrypts the overlay file, and then untars it over the rootfs.