Solved: correct mounting for Docker as a volume is -v /local/location/ncdata:/data/
You can also think of it as -v local:/container
Continuing the discussion from Nextcloudpi docker container does not recognize external storage:
NextcloudPi (19.0.3 running via Docker arm64) is simply not using the data directory I’ve mounted in Docker as ncdata:/home/user/microsd. Nextcloud is continuing to use the onboard partition instead. 
I’m attempting to use a spare MicroSD card for my data directory as ext4, which is fstab mounted by
UUID=redacted-values /home/user/microsd ext4 defaults 0
$ df -h
dev/mmcblk1p1 32G 60M 31G 1% /home/user/microsd
After mounting, www-data was granted ownership:group
$ sudo chown www-data:www-data -R /home/user/microsd/
drwxr-xr-x 3 www-data www-data 4096 Sep 27 11:47 microsd
Any ideas or tips appreciated on what is missing!