feat: add Dockerfile for Node.js backend #111

[jfernsio]

What was done

• Added Dockerfile that builds and runs the Node backend and documented how to run with docker run and env vars (closes Add Dockerfile for backend #111)

Changes

• Added a production-ready Dockerfile using multi-stage build with node:20-alpine for smaller & more secure image.
• Added .dockerignore to exclude unnecessary files.
• Updated README.md with:
  • How to build the Docker image
  • docker run command example
  • Table of required environment variables

Why

This makes it easy for anyone to run the Node backend consistently with Docker.

[vercel]

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