helpers to create mbr/uefi keys see:

• ./dokey.sh
• ./makengkey.sh

key layout

• gparted -> gpt partition, 1 partition fat32, flags boot+esp

• gdisk -> hybrid MBR

   gdisk /dev/mydisk
   	Command (? for help): r
   	Recovery/transformation command (? for help): h
   	Type from one to three GPT partition numbers, separated by spaces, to be added to the hybrid MBR, in sequence: 1
   	Place EFI GPT (0xEE) partition first in MBR (good for GRUB)? (Y/N): y
   	Set the bootable flag? (Y/N): y
   	Recovery/transformation command (? for help): w
   	Do you want to proceed? (Y/N): y

• install key

  docker-compose build
  docker-compose run --rm app
  mount /dev/sda1 usbmount
  ./makengkey.sh /app/usbmount
  umount usbmount

refs:

• https://github.com/ndeineko/grub2-bios-uefi-usb