Update the list of installed DUB packages automatically by checking the list of installed DUB packages from the Docker container

[wilzbach]

This should avoid the need of any manual updates in the future. If specific bumps or locks are required, this can be done entirely in the core-exec package. It's the single source of truth now.

It's all based on this simple idea to use an alternative entrypoint:

docker run --rm --entrypoint=/bin/cat dlangtour/core-exec:dmd /installed_packages