So, I found a report of someone else doing something similar with Kali Linux here: Playing around with other distros in toolbox - #8 by slt
In that case though, they appear to be making the toolbox container itself kali-based rather than running a container in a container. Am I misinterpreting what is being done? Which method is more likely to result in a stable system? The Kali Linux example appears to be based on a pre-built Kali Linux podman container from an upstream repository rather than something that is cobbled together from scratch. Basing the toolbox image on an official upstream container image seems like it might be at least a little more stable. Can something similar be done with your Ubuntu Linux idea?