Sorry for the confusion. I had not known that "JSON Schema" was a commonly used term like this.
Thank you for bringing this to my attention. I've just opened a pull request on the docker github repo to use a different word for the title of this section [1]. In the context of docker images, the term "Image JSON" is commonly used to mean the image metadata JSON file. This section was titled "Image JSON Schema" not to say that it is a "JSON Schema". Rather, I used the word "Schema" more generically here to mean that this section is an outline/description for the format of this file.
I also agree that this thread is not an appropriate place to discuss the docker image format. If you have any feedback or questions related to it, please open an issue on the github repository, ping me, and I'll gladly help.