feat: add docker-compose for node server mode with music volume mount

[subjec2change]

What changed

• Added docker-compose.yml using Dockerfile.server (Node server mode with caching proxy)
• Added music/ directory with gitkeep for mounting local music files
• Bumped package-lock.json (minor version)

How to use

# Pull your music folder and drop .mp3 files in it
mkdir -p music

# Build and run
docker compose up -d

# Access the app
open http://localhost:8080

Music files in ./music/ are mounted read-only into /app/server/music/ inside the container.
The server-side proxy handles the playlist and serves music at http://localhost:8080/music/.

[netbymatt]

I like how this helps to simplify the whole music situation. Can you also update the readme to match this new folder location?