feat: add Docker deployment - #28

Summary

• Add a multi-stage Docker image for the interactive CLI.
• Run the app as the non-root node user and document a bind mount that keeps downloads on the host.
• Exclude build output, dependencies, and the local downloads folder from the build context.

Refs #10.

Validation

• npm run prepublishOnly passed on the base source (10 tests, typecheck, and build).
• Built the Docker image successfully from PR head 72839c919515c8288b5351f2842014040eeee9a1 using Docker Engine 29.4.1 and the supplied multi-stage Dockerfile.
• The default image entrypoint prints --help successfully. An interactive docker run --rm -it starts the link prompt and exits cleanly on Ctrl+C.
• A container running as UID 1000 writes into a bind-mounted /home/node/Downloads; the resulting file is readable on the host. The bundled ffmpeg-static binary executes successfully in the runtime image.
• These are image, entrypoint, terminal, dependency, and mount smoke checks. A live third-party video download was not exercised.