add note about amd64 prereqs for building

vmdb2-recipes/amd64_trixie.yaml

@@ -63,6 +63,9 @@ steps:
 
   - mount: tag-root
 
+  # NOTE: gptmbr.bin is read from the HOST filesystem (not the target rootfs),
+  # so the `syslinux-common` package must be installed on the build machine:
+  #   sudo apt install syslinux-common
   - shell: |
       dd bs=440 count=1 conv=notrunc if=/usr/lib/syslinux/mbr/gptmbr.bin of="{{ output }}"
     root-fs: tag-root