Directly booting an ISO file
It is technically possible to boot an .iso file directly, without flashing it to a disk or device.
Using a virtual machine
The QEMU page shows how an ISO image and .apkovl customizations are booted with a virtual machine. This works very well with Proxmox as well - just attach the ISO and Alpine boots to RAM on startup. You can customize your .iso file by building a custom ISO image by following the instructions on How to make a custom ISO image with mkimage
Using an installed Bootloader
grub-imageboot
In addition to standard partitions or drives, the Debian package grub-imageboot allows booting .iso files placed in the /boot/images directory.
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syslinux
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