Busybox ash shell is more powerful than OSv --runscript option.
Use ash when fancy shell is required.

While busybox config file includes default options (e.g. nearly all
options are enabled), the usr.manifest doesn't try to create all
valid symlinks to busybox binary. Instead, only a few are included.

A small test script is provided (commented out in usr.manifest).
It can be run as:
  scripts/run.py -e "sleep 1.2; ash /cmd0.sh; echo Done"

You can also start an interactive shell with:
  scripts/run.py -e ash