[Bug] docker: fragmented usage explanations

[neteler]

Describe the bug

I see a need to improve and streamline the Docker-related documentation: Currently it is very fragmented and some sections are quite unexpected (like how to build a docker image in the main README.md).

What I found (there may be more)

• https://grass.osgeo.org/download/docker/
  • There are no usage instructions here. I think they should be there, or at least pointers to where to find explanations (see e.g. user question on grass-user about how to use the GRASS GIS docker images)
• https://github.com/OSGeo/grass/blob/main/README.md#docker
  • The docker instructions start with "Building a docker image" when it should start with "Using existing images". Move the building part somewhere else?
  • Note: this README is also shown on dockerhub
• https://github.com/OSGeo/grass/blob/main/docker/README.md
  • Here are some info bits as well, although they are missing what images we have or how to use them.
• Individual READMEs in the docker image subdirs with corresponding build instructions:
  • https://github.com/OSGeo/grass/blob/main/docker/debian/README.md
  • https://github.com/OSGeo/grass/blob/main/docker/testdata/README.md
  • https://github.com/OSGeo/grass/blob/main/docker/ubuntu/README.md
  • https://github.com/OSGeo/grass/blob/main/docker/ubuntu_wxgui/README.md

Expected behavior

Streamline all of this and lower the barrier.

Additional context

No strong opinion on what to put where -> happy to discuss here in this issue.

[wenzeslaus]

Individual READMEs in the docker image subdirs with corresponding build instructions

If these stay, the probably should use the source code, that would avoid the need for two nested commands in #4131. Why would anyone go all the way to deeply nested readme to get instructions how to check out the latest version? They can just use Git to switch if they want of they want to use source code. If they don't want to use source code, they are using an image from DockerHub.

Maybe also a single readme would be enough as the idea is the same every time. Or the readme should focus on what is unique about the image like getting the GUI working for the wxGUI one.