chore(docker): add Dockerfile for production image

[gouravjshah]

This PR adds a Dockerfile for building a production-ready image of the Node.js/Express front-end.

Notes:

• Base image: node:18-alpine
• Installs production dependencies only
• Runs the app as the non-root "node" user
• Exposes port 8079 (default in server.js)

Recommendations that are not included in this PR:

• Add a .dockerignore to reduce build context
• If native modules fail to build, consider switching to node:18-slim or installing build deps
• Ensure SESSION_REDIS and PORT are provided at runtime as needed