Add config file hydration and update README

[akshat-kumar-singhal]

Summary

• Add hydrateConfig function that replaces ${VAR} placeholders in a config file (set via CONFIG_FILE_PATH) with environment variable values at startup
• Add tests for config hydration covering happy path, missing vars, and edge cases
• Update README: document config hydration feature, fix website → static references, bump image tag to v0.0.8, replace EXPOSE 8000 with HTTP_PORT guidance, add CONFIG_FILE_PATH
  to example Dockerfile