Deprecation: PlayWithDocker

[lujobi]

PlayWithDocker has been deprecated. Move to:

• https://docs.docker.com/get-started/docker-concepts/building-images/
• https://docs.docker.com/get-started/workshop/

These are more or less the same exercises but on the polished Docker doc page and without an integrated playground. This is no problem as we have a suitable dev-container anyway.