Assuming you know the name of your container, or if you don’t run docker ps to list the processes running and copy the container name…
Use either docker run or use docker exec with the -i (interactive) flag to keep stdin open and -t to allocate a terminal.
docker exec -i -t container_name /bin/bash
Or to run one or many commands:
docker exec -ti my_container sh -c "echo a && echo b"