Any image allowed by default

dockerspawner is a tool to spawn JupyterHub single user servers in Docker containers. Users of JupyterHub deployments running DockerSpawner starting with 0.11.0 without specifying `DockerSpawner.allowed_images` configuration allow users to launch _any_ pullable docker image, instead of restricting to only the single configured image, as intended. This issue has been addressed in commit `3ba4b665b` which has been included in dockerspawner release version 13. Users are advised to upgrade. Users unable to upgrade should explicitly set `DockerSpawner.allowed_images` to a non-empty list containing only the default image will result in the intended default behavior.