First, Q&D version that creates an ISO

Makefile

@@ -4,8 +4,8 @@ ALL_SNAPS = $(EXTRA_SNAPS) evince firefox gnome-calculator gnome-characters gnom
 
 all: pc.img
 
-bootable: ubuntu-core-desktop-22-amd64.img
-bootable-dangerous: ubuntu-core-desktop-22-dangerous-amd64.img
+bootable: ubuntu-core-desktop-22-amd64.iso
+bootable-dangerous: ubuntu-core-desktop-22-dangerous-amd64.iso
 
 define build_img =
 rm -rf img${1}
@@ -46,6 +46,9 @@ ubuntu-core-desktop-22-amd64.img: pc.img.xz image/core-desktop.yaml.in image/ins
 ubuntu-core-desktop-22-dangerous-amd64.img: pc-dangerous.img.xz image/core-desktop.yaml.in image/install-sources.yaml.in
 	$(call build_bootable_img)
 
+ubuntu-core-desktop-22-%.iso: ubuntu-core-desktop-22-%.img
+	./create_iso.sh $<
+
 clean:
 	sudo rm -rf img
 	sudo rm -rf output

create_iso.sh

@@ -0,0 +1,132 @@
+#!/bin/sh
+
+# Process obtained from https://itnext.io/how-to-create-a-custom-ubuntu-live-from-scratch-dd3b3f213f81
+
+# we expect a .xz compressed image (see below where we decompress it)
+ORIGINAL_DISK_IMAGE=$1
+DISK_IMAGE=$PWD/output/disk.img
+
+# where to mount the disk image
+CHROOT=$PWD/output/mnt
+
+cp -a ${ORIGINAL_DISK_IMAGE} ${DISK_IMAGE}
+
+cd output
+# decompress the image
+xz -v -d ${DISK_IMAGE}
+cd ..
+
+mkdir -p $CHROOT
+# since we know that the partition with the data is the third one, we use awk to extract the start sector
+sudo mount -o loop,offset=$(expr 512 \* $(fdisk -l ${DISK_IMAGE} |grep img3 | awk '{print $2}')) ${DISK_IMAGE} $CHROOT
+
+# we prepare the chroot environment to ensure that everything works inside..
+sudo mount -o bind /dev ${CHROOT}/dev
+sudo mount -o bind /dev/pts ${CHROOT}/dev/pts
+sudo mount -o bind /proc ${CHROOT}/proc
+sudo mount -o bind /sys ${CHROOT}/sys
+sudo mount -o bind /run ${CHROOT}/run
+# install the required packages
+sudo chroot ${CHROOT} apt install -y casper
+# and, if needed, open a bash shell to do manual checks
+# chroot ${CHROOT} /bin/bash
+sudo umount ${CHROOT}/run
+sudo umount ${CHROOT}/sys
+sudo umount ${CHROOT}/proc
+sudo umount ${CHROOT}/dev/pts
+sudo umount ${CHROOT}/dev
+
+rm -rf $PWD/image2
+
+mkdir -p $PWD/image2/casper
+mkdir -p $PWD/image2/isolinux
+mkdir -p $PWD/image2/install
+
+mv $CHROOT/cdrom/casper/* $PWD/image2/casper/
+
+sudo cp $CHROOT/boot/vmlinuz-**-**-generic image2/casper/vmlinuz
+sudo cp $CHROOT/boot/initrd.img-**-**-generic image2/casper/initrd
+
+touch image2/ubuntu
+
+cat <<EOF > image2/isolinux/grub.cfg
+
+search --set=root --file /ubuntu
+
+insmod all_video
+
+set default="0"
+set timeout=3
+
+menuentry "Install Ubuntu Core Desktop" {
+   linux /casper/vmlinuz boot=casper quiet splash ---
+   initrd /casper/initrd
+}
+EOF
+
+sudo mksquashfs $CHROOT image2/casper/filesystem.squashfs
+
+printf $(sudo du -sx --block-size=1 $CHROOT | cut -f1) > image2/casper/filesystem.size
+
+# we are done with the original disk image
+sudo umount ${CHROOT}/sys/firmware/efi/efivars
+sudo umount ${CHROOT}/sys
+sudo umount ${CHROOT}
+rmdir ${CHROOT}
+
+cd $PWD/image2
+
+grub-mkstandalone \
+   --format=x86_64-efi \
+   --output=isolinux/bootx64.efi \
+   --locales="" \
+   --fonts="" \
+   "boot/grub/grub.cfg=isolinux/grub.cfg"
+
+(
+   cd isolinux && \
+   dd if=/dev/zero of=efiboot.img bs=1M count=10 && \
+   sudo mkfs.vfat efiboot.img && \
+   LC_CTYPE=C mmd -i efiboot.img efi efi/boot && \
+   LC_CTYPE=C mcopy -i efiboot.img ./bootx64.efi ::efi/boot/
+)
+
+grub-mkstandalone \
+   --format=i386-pc \
+   --output=isolinux/core.img \
+   --install-modules="linux16 linux normal iso9660 biosdisk memdisk search tar ls" \
+   --modules="linux16 linux normal iso9660 biosdisk search" \
+   --locales="" \
+   --fonts="" \
+   "boot/grub/grub.cfg=isolinux/grub.cfg"
+
+cat /usr/lib/grub/i386-pc/cdboot.img isolinux/core.img > isolinux/bios.img
+
+sudo /bin/bash -c "(find . -type f -print0 | xargs -0 md5sum | grep -v "\./md5sum.txt" > md5sum.txt)"
+
+sudo xorriso \
+   -as mkisofs \
+   -iso-level 3 \
+   -full-iso9660-filenames \
+   -volid "Ubuntu Core Desktop" \
+   -output ${DISK_IMAGE%.*}.iso \
+   -eltorito-boot boot/grub/bios.img \
+      -no-emul-boot \
+      -boot-load-size 4 \
+      -boot-info-table \
+      --eltorito-catalog boot/grub/boot.cat \
+      --grub2-boot-info \
+      --grub2-mbr /usr/lib/grub/i386-pc/boot_hybrid.img \
+   -eltorito-alt-boot \
+      -e EFI/efiboot.img \
+      -no-emul-boot \
+   -append_partition 2 0xef isolinux/efiboot.img \
+   -m "isolinux/efiboot.img" \
+   -m "isolinux/bios.img" \
+   -graft-points \
+      "/EFI/efiboot.img=isolinux/efiboot.img" \
+      "/boot/grub/bios.img=isolinux/bios.img" \
+      "."
+
+cd ..
+mv ${DISK_IMAGE%.*}.iso ${ORIGINAL_DISK_IMAGE%.*}.iso