Docker: Fixes annoying message at restart, makes the image easier to use in non root mode + Cleanup useless files

[fathzer]

Some enhancements of the Docker image:

• Makes config.yml link during image build, not at runtime.
• Removes useless ARG in Docker file.
• Removes useless files in the image (everything but copy_files.sh in docker directory)

Type of pull request:

• Bug fix
• Feature
• Other

Description:

The main change is to make the config.yml link during the build.
The advantages are:

• No more annoying ln: /lichess-bot/config.yml: File exists message in log when a container is restarted.
• More importantly: for those, like me, who don't need custom extra_game_handlers.py or extra_game_handlers.py, this allows again (as in version 2024.5.1.2) to run a container based on this image as a non-root user without digging into the source code to guess what permissions are required to prevent the launch from failing with a permission error.
  Since the link is created during construction, no permissions are needed to use it at runtime.
  PS: Running a container as root on a production environment is a well-known worst practice. Copying extra_game_handlers.py or extra_game_handlers.py into the container file system still requires guessed permissions.

Checklist:

• I have read and followed the contribution guidelines.
• I have added necessary documentation (if applicable).
• The changes pass all existing tests.