Docker - Cassandra with Authentication
The only option that I can think of (other than making your own version of the image and updating that docker-entrypoint.sh, as you suggested) is to provide your own cassandra.yaml in a bind mount. For example:
$ docker run -v /path/to/config:/etc/cassandraWhere /path/to/config is a directory containing your cassandra.yaml. Make any adjustments you like to the copy of cassandra.yaml on the host, including your auth changes. To ensure consistency in the configuration, be sure your copy of cassandra.yaml matches the version embedded in the docker image.