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.github/instructions/containerization.md

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+# Containerization Instructions
+
+## Scope
+- Applies to Dockerfiles, `docker-compose.yml`, and make targets that run services.
+
+## Rules
+- Keep service startup deterministic: API, chat, webrtc, and seed images should build independently.
+- Avoid embedding secrets in Dockerfiles; rely on `.env` and compose environment wiring.
+- Preserve fast local iteration paths (`dev-local`, seed commands) when changing build contexts.
+- Keep images minimal and purpose-specific (`Dockerfile`, `Dockerfile.chat`, `Dockerfile.webrtc`, `Dockerfile.seed`).
+- If ports or health behavior change, update compose and README usage notes together.
+
+## Operational Notes
+- Prefer non-destructive changes to compose volumes and DB state in dev.
+- When build/runtime differs between local and CI, document the reason in PR notes.