FYI: If you are using the general gazebo 9 tags from the Official Library on Docker Hub and are expecting xenial as the base image, you should know an PR has been created for migrating to bionic and should land in about a week. You can find more about this from the github ticket below:
Note the additional gazebo tags, such as libgazebo9-xenial, that specify the code names with respect to the Ubuntu base images used. This should provide docker users a suitable migration path while we continue to maintain long term support and updating the general set of tags to track the latest supported LTS base images. So if your application is also sensitive to ubuntu release in addition to gazebo version, consider using the full code named tags for your projects.