Improve docker politics

[mjanez]

Dockerfile and runtime improvements:

• Added pdm.lock to the Docker build context to ensure consistent dependency installation.
• Changed the entrypoint to use pdm run and point to src/main.py instead of running docs2md.py directly, aligning the container startup with the project's main script and dependency management best practices.

Docker Compose enhancements:

• Updated the docker-compose.yml to use a prebuilt image (ghcr.io/mjanez/docs2md:latest) with a pull_policy of missing, improving deployment consistency and startup speed.