How to make changes to the disk/root filesystem of a CheriBSD instance

Hi all,

I am currently running the Morello FVP on a CheriBSD instance, built with the cheribuild script. 

I added a test file (helloworld.txt) to the root filesystem of the machine under ~/cheri/output/rootfs-morello-purecap/root

I then rebuilt the machine and disk image with ./cheribuild.py run-fvp-morello-purecap -d

I assumed this would take into account the changes I've made, and would rebuild the machine/disk image to include them. However, when going to /root on the CheriBSD instance after rebuilding, the file isn't there. It has also been removed from the host system under ~/cheri/output/rootfs-morello-purecap/root

Why is this happening? And what is the best way to make amendments to a CheriBSD instance, such as adding files, so that when the machine is rebuilt/booted with the cheribuild script they are included?

Thanks!