If you are running docker 1.3 or above, you should use: docker exec -it CONTAINER COMMAND to run COMMAND within the container. You can easily create a function to ease the thing and run bash by default:

dk-enter() {
    docker exec -it "$1" "${2:-/bin/bash}"
}

Otherwies, you can easily open a shell in a running docker container with the following commands:

• Get the container pid: docker inspect -f {{.State.Pid}} <container-id>
• Connect: nsenter --target <pid> --mount --uts --ipc --net --pid