As you said you are new to NC, I’d suggest read a bit about how it stores data, and how you can move data to another drive here.
With docker, once the volume is mounted to the system, I’d use -v to mount NC container /data folder to the volume as exampled from here
-v /media/myvolume/ncdata:/data
My Docker knowledge is minimal, only use it for testing occasionally.
Afaik www-data needs ownership of /ncdata