add note about amd64 prereqs for building

README.md

@@ -4,6 +4,18 @@
 
 Tool for building Butter Box disk images.
 
+## Host prerequisites
+
+The build host needs `vmdb2` and, for the amd64 image, the `syslinux-common`
+package — the amd64 recipe writes GPT protective-MBR boot code
+(`/usr/lib/syslinux/mbr/gptmbr.bin`) directly to the output image from the host
+filesystem, so this package must be present on the build machine (not just
+inside the target image):
+
+```sh
+sudo apt install syslinux-common
+```
+
 ## Usage
 
 To build a butter image, run:

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