ADR-008: CI Base Image

Publishable summary of this architecture decision.

Decision

Do not maintain a large custom CI image by default. GitHub Actions should use native runners and official setup actions where possible.

• PHP via shivammathur/setup-php.
• Node via actions/setup-node.
• Composer with workflow cache.
• Specific tools installed on demand.
• Custom images only when a critical recurring stack justifies them.

Moodle and MkDocs may need explicit transitional workflow support, without making a heavy image the global default.