Add docker support

[beeltec]

Add Docker Support with All External Binaries

Summary

This PR adds Docker support for gamdl, providing a containerized environment with all required and optional external binaries pre-installed. Users can run gamdl without needing to manually install FFmpeg, mp4decrypt, MP4Box, or N_m3u8DL-RE on their system.

Changes

New Files

• Dockerfile: Multi-stage build that creates an optimized image with all dependencies
• .dockerignore: Excludes unnecessary files from the Docker build context

Modified Files

• README.md: Added Docker usage documentation with examples

Technical Details

Dockerfile Architecture

The Dockerfile uses a 4-stage multi-stage build for optimal image size:

Stage	Purpose	Base Image
builder	Install gamdl from PyPI	python:3.11-slim
bento4-builder	Compile mp4decrypt from source	python:3.11-slim
gpac-builder	Compile MP4Box from source	python:3.11-slim
Final	Runtime image	python:3.11-slim

Included Binaries

Binary	Source	Purpose
FFmpeg	Debian packages	Remuxing, format conversion
mp4decrypt	Built from Bento4	Widevine DRM decryption
MP4Box	Built from GPAC	Alternative remuxing
N_m3u8DL-RE	Latest release from GitHub	Fast HLS downloading

Security Considerations

• Uses --no-install-recommends to minimize attack surface
• Cleans up apt lists after installation
• Uses python:3.11-slim base for smaller footprint
• .dockerignore prevents sensitive files (cookies, .env) from being copied into the build context

Usage Examples

Basic usage:

docker build -t gamdl .
docker run --rm \
  -v /path/to/cookies.txt:/app/cookies.txt:ro \
  -v /path/to/output:/app/music \
  gamdl --cookies-path /app/cookies.txt --output-path /app/music "https://music.apple.com/..."

With N_m3u8DL-RE downloader:

docker run --rm \
  -v ./cookies.txt:/app/cookies.txt:ro \
  -v ./music:/app/music \
  gamdl --cookies-path /app/cookies.txt --output-path /app/music --download-mode nm3u8dlre "https://music.apple.com/..."

With config persistence:

docker run --rm \
  -v /path/to/cookies.txt:/app/cookies.txt:ro \
  -v /path/to/output:/app/music \
  -v /path/to/config:/root/.gamdl \
  gamdl --cookies-path /app/cookies.txt --output-path /app/music "https://music.apple.com/..."

Test Results

• Build Docker image on amd64: docker build -t gamdl .
• Build Docker image on arm64: docker build -t gamdl .
• Verify all binaries are present: docker run --rm gamdl --help
• Test basic song download with ffmpeg remuxing
• Test download with --download-mode nm3u8dlre
• Test download with --remux-mode mp4box
• Verify mp4decrypt works for DRM content
• Test config persistence across container restarts