Nextcloud - Official Image | Docker Hub
https://hub.docker.com/_/nextcloudThe Nextcloud installation and all data beyond what lives in the database (file uploads, etc.) are stored in the unnamed docker volume volume /var/www/html. The docker daemon will store that data within the docker directory /var/lib/docker/volumes/.... That means your data is saved even if the container crashes, is stopped or deleted.
Nextcloud - Official Image | Docker Hub
hub.docker.com › _ › nextcloudFirst use. When you first access your Nextcloud, the setup wizard will appear and ask you to choose an administrator account username, password and the database connection. For the database use db as host and nextcloud as table and user name. Also enter the password you chose in your docker-compose.yml file.