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source: http://askubuntu.com/a/507009/89694

$ sudo docker attach 665b4a1e17b6 #by ID
or
$ sudo docker attach loving_heisenberg #by Name
$ root@665b4a1e17b6:/# 

UPDATE: (docker >= 1.3) Thanks to WiR3D user who suggested another way to get container’s shell. If we use attach we can use only one instance of shell. So if we want open new terminal with new instance of container’s shell, we just need run the following:

$ sudo docker exec -i -t 665b4a1e17b6 bash #by ID
or
$ sudo docker exec -i -t loving_heisenberg bash #by Name
$ root@665b4a1e17b6:/#