Add Docker container installation option

[choopachuck]

Feature Request: Official Docker container support

Description

It would be very useful to have an official Docker way to run chatter.

While the project works great as a Rust CLI tool, many users (especially in server/self-hosted setups, CI/CD environments, or machines without Rust installed) prefer running applications via Docker containers.

Suggested features

• Official Docker image (ghcr.io/isa/chatter)
• Dockerfile in the repository
• docker-compose.yml example
• Multi-arch support (linux/amd64, linux/arm64)
• Documentation in README.md with usage examples

Why this matters

• Easier installation on VPS and Linux servers
• No need to install Rust toolchain
• Better integration into automation scripts and workflows
• Simplified usage on macOS via Docker Desktop
• Easy GPU support in the future (--gpus all)

Example usage

# Basic usage
docker run --rm -v "$(pwd):/data" ghcr.io/isa/chatter generate \
  --file /data/input.md \
  --profile storyteller \
  -o /data/output.mp3

# With persistent configuration
docker run --rm \
  -v chatter_data:/root/.chatter \
  -v "$(pwd):/data" \
  ghcr.io/isa/chatter generate \
  --file /data/input.md \
  --profile storyteller \
  -o /data/output.mp3

I'm happy to help write and test the Dockerfile if needed.