Here is a quick fix for getting netboot.xyz to use your local menus instead of always downloading the menus from Github

By default the live endpoint for live assets is pulled from https://github.com/netbootxyz. This can cause issues when you are making custom menus. To fix this open up the boot.cfg and comment outline 14 and add an endpoint pointing to your netboot.xyz instance running in your network. In my case I am not using the default port 80 and the container is pointing to 8282

# set location of custom netboot.xyz live assets
# set live_endpoint https://github.com/netbootxyz
set live_endpoint http://10.0.0.10:8282

This was annoying me to no end until I found the fix here: Zakna’s comment in Unraid forums