When I enter a running docker with "docker exec -ti mydockerinstance /bin/bash"

tty returns: not a tty and tmux doesn't work.

I tried several things from #728:
docker exec -ti mydockerinstance sh -c "exec >/dev/tty 2>/dev/tty </dev/tty && /usr/bin/tmux -s /bin/bash"
getty tty
but nothing succeded.