You launched your Docker container with
sudo docker –run –I –t –name grey_cat centos /bin/bash
This creates the container name “grey_cat” from base image “centos” and ran the bash shell inside it, the container’s is presented.
Once you are done with the container you type “exit” which stop the container and get back to you host shell.
So what happened to container? The container still exists; we can show a list of current containers using the docker ps -a command
To start the stopped container
sudo docker start grey_cat
This start the container with the same options when it was launched with docker run command.
To attach to the interactive session we can use attach command
sudo docker attach grey_cat
You might need to Enter to get the command prompt.