Run ycqlsh to connect to the service as follows:
$ docker exec -it yb-tserver-n1 /home/yugabyte/bin/ycqlsh yb-tserver-n1
Connected to local cluster at 127.0.0.1:9042.
[ycqlsh 5.0.1 | Cassandra 3.9-SNAPSHOT | CQL spec 3.4.2 | Native protocol v4]
Use HELP for help.
ycqlsh>
Run a YCQL command to verify it is working.
ycqlsh> describe keyspaces;
system_schema system_auth system
ycqlsh>