How to make a custom ISO image
ISO remastering
There are third party scripts to re-build completely custom images, like
(works to only make a disk based VM image)
and there is Alpine's original
(creates a diskless ISO file - usually what you want for QEMU and Proxmox)
An alternative
It's also possible to commit and load ISO image customizations to and from a writable partition when booting in Diskless Mode.