Feature: Add Dockerfile with multi-stage Node.js build

[RetroHoarder]

• Uses Node 18-slim for build and runtime
• Installs dependencies and builds TypeScript in build stage
• Copies only dist and package files to production image
• Suitable for running the igir CLI tool in Docker

[github-actions]

👋 Welcome

Thank you for your first pull request, @RetroHoarder! If you haven't yet, please familiarize yourself with Igir's contribution guidelines.

Some GitHub Actions CI workflows may not automatically run for you due to GitHub's security best practices, so a maintainer may need to manually approve the workflows to run. As a result, it is important to make sure tests pass locally before submitting a pull request to help ensure a fast review. Thank you!

Comment generated by the GitHub First Interaction workflow.

[github-actions]

🧪 Branch testing instructions

This pull request can be tested locally with the following command:

npm exec --yes -- "github:RetroHoarder/igir#main" [commands..] [options]

Comment generated by the Node.js Branch Exec workflow.