Make phpunit Docker image fully self-contained

[hokoo]

Summary

• rebuild Dockerfile.phpunit around a new entrypoint so it now installs WordPress core/tests, Composer, and a MariaDB instance that boots in-container for both the plugin and WordPress suites
• remove the phpunit service dependency on external containers, update the README, and teach the optional GitHub Actions workflow to rely solely on the new universal image

Testing

• not run (CI/Docker-only changes)

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Codex Task