To update this case, for now I detected that only you can upload non config files, because script process will remove files in the process to setup.

inside customize.json you need to add for example:

"files": "imagebuilder/3.1/openwrt-imagebuilder-ar71xx-generic_3.1/files" 

and this folder “files” is the root tree to copy files, in my case I created

imagebuilder/3.1/openwrt-imagebuilder-ar71xx-generic_3.1/files/etc/config/test

but I don’t know how to change config templates.