Seems that rt-cli is not working because in the container, bitcoin-cli is no working.
This is what I can see in the CLI for the container instance:
root@a410111b116e:/# bitcoin-cli
bash: bitcoin-cli: command not found
And, when executing the instructions about Use REGTEST aliases to start container, view logs, run cli command, stop container
rt-cli help
error code: -32601
error message:
Method not found