So running unprivileged buildah inside a toolbox is running unprivilged buildah inside a rootless podman container which I don’t think is well supported right now. You might want to consider running podman build from outside a toolbox as a first step.
I also found:
- rootless: overlay mounts broken with 1.19 · Issue #3051 · containers/buildah · GitHub
- Build container with unprivileged mode in Kubernetes · Issue #2554 · containers/buildah · GitHub
- Error: executable file `ls` not found in $PATH: No such file or directory: OCI runtime command not found error
that might help.