Add MariaDB as a first-class database backend #142

Part of #140 and LibreSign/libresign#8969.

Context

nextcloud-docker-development currently provides explicit MySQL and PostgreSQL database services.

LibreSign's Behat compatibility matrix also exercises MariaDB 10.6 and 10.11.

The parent worker contract defines database selection as:

DB_TYPE=sqlite|mysql|mariadb|pgsql

This issue implements the mariadb branch of that contract.

Goal

Add MariaDB as a first-class selectable database backend in nextcloud-docker-development.

Requirements

• Add a MariaDB service to the database service definitions rather than treating MariaDB as an undocumented MySQL alias.
• DB_TYPE=mariadb must select the MariaDB service and MySQL-compatible Nextcloud database driver.
• Keep DB_HOST as the connection/service host; do not overload it as the backend selector.
• Support programmatic MariaDB version selection without editing Compose YAML.
• Initial supported versions for this work are MariaDB 10.6 and 10.11.
• Do not use an unpinned latest image for CI selection. Follow the repository's existing image-pinning/update conventions for the supported versions.
• Preserve per-clone/per-worker database storage.
• Preserve readiness checks before Nextcloud installation starts.
• Do not remove or silently replace the existing MySQL backend.

SQL mode contract

LibreSign's current MariaDB Behat workflow enables ONLY_FULL_GROUP_BY.

Provide a generic caller-controlled mechanism for database initialization/session/global SQL configuration that can deterministically enable ONLY_FULL_GROUP_BY before the application test suite runs.

Do not hard-code a LibreSign-named switch.

The automated test for this issue must query @@sql_mode and prove ONLY_FULL_GROUP_BY is active when requested.

Tests

Add automated coverage proving:

• DB_TYPE=mariadb with MariaDB 10.6 starts and installs Nextcloud;
• DB_TYPE=mariadb with MariaDB 10.11 starts and installs Nextcloud;
• the MariaDB version is selected programmatically;
• requested ONLY_FULL_GROUP_BY configuration is actually active;
• two MariaDB workers do not share database state;
• existing MySQL and PostgreSQL backends continue to work.

Documentation

Document:

• DB_TYPE=mariadb;
• MariaDB version selection;
• the supported versions introduced by this issue;
• the generic mechanism for optional SQL initialization/configuration.

Done when

• DB_TYPE=mariadb is supported.
• MariaDB 10.6 and 10.11 are selectable without editing Compose files.
• Nextcloud autoinstall works against each supported MariaDB version.
• Requested ONLY_FULL_GROUP_BY configuration is proven active by an automated test.
• MariaDB database state remains isolated per worker.
• Existing MySQL/PostgreSQL behavior is not regressed.
• Automated tests and documentation cover the backend.