This image is based on the popular Alpine Linux project, available in the alpine official image. Alpine Linux is much smaller than most distribution base images (~5MB), and thus leads to much slimmer images in general.
The OpenJDK port for Alpine is not in a supported release by OpenJDK, since it is not in the mainline code base. It is only available as early access builds of OpenJDK Project Portola. See also this comment. So this image follows what is available from the OpenJDK project's maintainers.
What this means is that Alpine based images are only released for early access release versions of OpenJDK. Once a particular release becomes a "General-Availability" release, the Alpine version is dropped from the "Supported Tags"; they are still available to pull, but will no longer be updated.