There is no longer Docker support for NextcloudPi, so adios. 
You’ll need to move to something else, such as All-In-One image or any other Nextcloud Docker image.
To use Docker you’ll want to read documentation on how it works. See https://docs.docker.com/
Exactly, welcome to the wider world of Linux and Docker. You’ll need to learn some basics, but keep in mind you can always search the forum or read documentation or search the interwebs. You’ll want to learn what volumes are in Docker and how permissions work in linux.