feat: support custom image tags

[Anshumancanrock]

Summary

This PR adds support for custom Docker image tags for Translator and JDC, plus a pull policy so image behavior is predictable across environments.

Closes #92

What changed

• Added optional images config to setup data (translator_image, jdc_image, pull_policy).
• Added a small resolver to apply config in this order: setup payload -> env vars -> defaults.
• Updated Docker startup flow to respect pull policy (always, if-not-present, never).
• Added focused tests for image config resolution.
• Documented the new environment variables in the README.

Why

This makes testing and deployment easier (pinning specific image tags) without changing default behavior for existing users.

Validation

• npm run build --prefix server
• npm run test --prefix server