When creating a container using the docker run command, the -it or --interactive and -t or --tty flags can be used to establish an interactive shell into the container. In this example, a container is created using the foo:latest image, and the bash shell will be used. Notice in this example that after issuing the docker run command with the --interactive option, a prompt appears to issues commands within the container.

[john.doe@docker ~]# docker run --interactive --tty foo:latest /bin/bash
john.doe@f085285ed20c:/#

Or, you could the /bin/bash command building the image from a Dockerfile.

CMD ["/bin/bash"]