Run ycqlsh to connect to the service.

You can do this as shown below.

$ docker exec -it yb-tserver-n1 /home/yugabyte/bin/ycqlsh yb-tserver-n1
Connected to local cluster at
[ycqlsh 5.0.1 | Cassandra 3.9-SNAPSHOT | CQL spec 3.4.2 | Native protocol v4]
Use HELP for help.

Run a YCQL command to verify it is working.

ycqlsh> describe keyspaces;
system_schema  system_auth  system