[DRAFT] Docker compose healthcheck

[fe51]

Draft PR, to review after #329

This PR introduces the following modifications :

• engine: wrap run.py with tee to log output to engine.log; healthcheck greps for "confidence" to confirm inference is running ("confidence" is so far always in logs attesting engine is running)
• pyro-camera-api: healthcheck hits GET /health (port 8081); add curl to the Dockerfile to support it
• image versioning: replace hardcoded latest tags with ${PYRO_ENGINE_VERSION:-latest} on both services, allowing pinned deploys via .env
• docs: update README env vars section; add .env.example with all required variables documented