We do not have access to a local GitHub installation in order to test this, which is why we do not have documentation for it. However, you should be able to use two existing articles in the official Apicurio documentation to help.
Here is the article about how to configure a local GitLab instance:
You'll obviously want to extrapolate from the information in that article to GitHub instead of GitLab. To help with that, there is some good information about how to configure the Keycloak side of things here:
The Apicurio configuration is pretty easy, it's just setting some system properties:
The values above are defaults - obviously used for integrating with the public instance of GH. You'll want to set BOTH properties to appropriate values for your local instance.
The trickier part is configuring your local Keycloak. This requires some configuration in Keycloak **AND** some configuration in your local GitHub instance. The configuration needed is documented in the second link I included above.
Let me know if you run into any issues.
-Eric