automatically generate readme for docker hub

[McPringle]

The README for the Docker Hub repository is limited to 2'000 characters. Actually there is a separate file: README_DOCKERHUB.md

A better solution (to avoid duplicate docs) would be to generate this file automatically by extracting the relevant paragraphs from the main README.md file. I found a GitHub action which looks like an easy solution:

https://github.com/sean0x42/markdown-extract

This should be added to the Docker Nightly action as the second last step just before the Docker Hub Description step.