vagrant@ubuntu-14:~/weave$ docker run -di busybox
1b91aa935eac5471b4030594fb30b86471107a60c705385b3ef7310ec4e3bcf4
vagrant@ubuntu-14:~/weave$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
1b91aa935eac        busybox:latest      "/bin/sh"           2 seconds ago       Up 2 seconds                            condescending_fermat
vagrant@ubuntu-14:~/weave$ docker exec 1b91aa935eac true # this works fine
vagrant@ubuntu-14:~/weave$ echo $?
0
vagrant@ubuntu-14:~/weave$ docker exec -i 1b91aa935eac true # this hangs until you hit ENTER

vagrant@ubuntu-14:~/weave$ echo $?
255