I figured out the problem. The IP address must not be in quotes. The quotes surrounding the CASSANDRA_HOST value in this response here threw me off - Temporalio/admin-tools using docker to create cassandra schema - #17 by markmark
To be clear, the command that works is
~$ sudo docker run --rm -it \
> -e CASSANDRA_HOST=xx.xx.xx.xx -e CASSANDRA_PORT=9042\
> --entrypoint /bin/sh temporalio/admin-tools -c\
> "temporal-cassandra-tool create-Keyspace -k temporal"
where xx.xx.xx.xx is an IP address and not a hostname.