Improve installer and add docker compose support

[ChadOhman]

Summary

• harden the Ubuntu installer with repository-root awareness, readiness checks, and clearer reporting while keeping auto web/database detection
• add environment-aware database configuration and a Docker Compose stack that supports either MySQL or MariaDB with seeded data
• refresh contributor guidance and README to document the installer expectations and new container workflow

Testing

• bash -n install_ubuntu.sh
• php -l db/connect.php

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Codex Task