whitelist.py errors for Docker installation (without Python3) and /etc/pihole on different directory

Running pihole 5.1.2 in docker container.
/etc/pihole is /home/pi/IOTstack/volumes/pihole/etc-pihole/
python 3.7.3 is installed

The installation instructions are interpreted to use the following to install, based upon how this pihole is running, after performing a successful git clone.
> sudo python3 whitelist/scripts/whitelist.py --dir /home/pi/IOTstack/volumes/pihole/etc-pihole/ --docker

The script starts.
Result is:

>This script will download and add domains from the repo to whitelist.
> All the domains in this list are safe to add and does not contain any tracking or adserving domains.
>
> [i] Pi-hole path exists
> [i] Write access to /home/pi/IOTstack/volumes/pihole/etc-pihole/ verified
> Traceback (most recent call last):
> File "whitelist/scripts/whitelist.py", line 141, in
> if os.path.isfile(gravity_db_location) and os.path.getsize(gravity_db_location) > 0 and getPiholeVersion() >= 51:
> File "whitelist/scripts/whitelist.py", line 61, in getPiholeVersion
> pihole_version = subprocess.Popen(['pihole', 'version', '-p', '-c'], stdout=subprocess.PIPE)
> File "/usr/lib/python3.7/subprocess.py", line 775, in __init__
> restore_signals, start_new_session)
> File "/usr/lib/python3.7/subprocess.py", line 1522, in _execute_child
> raise child_exception_type(errno_num, err_msg, err_filename)
> FileNotFoundError: [Errno 2] No such file or directory: 'pihole': 'pihole'