There are two ways to achieve that, the first and most known is the following:
docker attach --sig-proxy=false <container>
This one literally attaches your bash to the container bash, meaning that if you have a running script, you will see the result.
To detach, just type: Ctl-P Ctl-Q
But if you need a more friendly way and to be able to create new bash instances, just run the following command:
docker exec -it <container> bash