Add Docker image support for Dragon Bandwidth Buddy - #1

Motivation

• Provide a simple containerized deployment path so the self-hosted SABnzbd dragon dashboard can be run as a Docker image on LAN hosts.
• Keep SABnzbd credentials server-side and make it easier to start the service with standard env vars (SAB_BASE_URL, SAB_API_KEY, PORT).

Description

• Add a Dockerfile based on node:20-alpine that sets WORKDIR /app, copies the repository, exposes port 4173, and runs node server.js.
• Add a .dockerignore to reduce build context and exclude git metadata and common noisy files.
• Update README.md with Docker build and run instructions and required environment variables for the SABnzbd proxy.

Testing

• Ran node --check server.js and node --check app.js, both checks passed.
• Attempted docker build -t dragon-bandwidth-buddy:test . but the Docker CLI is not available in this environment, so the image build could not be validated here (error: bash: command not found: docker).

Codex Task