Rework how to extend this image

[BWibo]

We are moving away from the initial idea to add folders for images, that extend our base-image. Extension images should live in separate repos in the future. This repo will only maintain sddi-base, and sddi image flavor in the future.
We want to provide a template folder with Github workflows and Dockerfiles, that can be used as a starting point for extending this image.

Todos

• Rework docs to describe the new approach
• Provide template folder structure for extending this image
• Rework build workflows, to work out of the box wi´hen starting a fresh Github repo from the template folder