It seems like there are two ways to add documentation to a docker image:
You can add a readme.md in the root folder (where your docker file is located) and this is meant to be parsed by the dockerhub automated build system.
The second way is by using the manifest https://docs.docker.com/docker-hub/publish/publish/#prepare-your-image-manifest-materials But the documentation doesn't really explain well how to annotate the manifest file for an image. Also it looks like the manifest command is considered experimental.
What is the recommended way of documenting a docker image?
Personally i prefer not having to add documentation when the container is being built, i would much rather a file in the source control. However the md file method seems to have minimal support.