[seL4] Easy way to edit VM guest file system

Hi, I'm learning about this project: https://github.com/seL4/sel4webserver

and would like to edit the guest filesystem. Digging around I found some cpio files that looks to me like may be the right place (output_overlay_rootfs.cpio, rootfs.cpio) but I'm not sure. So, as far I understand in theory I can just edit them and the execute "ninja" again. Before going that way, can anyone confirm this is the easiest/right way?

Thanks,