For local development, run ./build/composer-install.sh && php composer.phar install && php bin/console doctrine:database:create && php bin/console doctrine:migrations:diff && php bin/console doctrine:migrations:migrate to create the SQLite database which needs to reside on the host and be bind mounted into the container so data persists across rebuilds. See the Symfony doctrine migrations documentation for more information. To fill the database with some test data, run cat ./build/default-templates.sql | sqlite3 ./var/data.db and cat ./build/test-data.sql | sqlite3 ./var/data.db.
In production we use a MySQL database which should never be touched unless you know what you're doing. If you know what you're doing and need to change the database schema, run php bin/console doctrine:migrations:diff && php bin/console doctrine:migrations:migrate in the container (where composer is already installed).