Running the docker server changes permissions of /dev/ptmx, crashing tmux

[durka]

After running the cpud_test example from the README, the permissions of /dev/ptmx are incorrect:

$ ls -l /dev/pts                                                                                                                                                                                                                                                                                                
total 0                                                                                                                                                                                                                                                                                                                        
[snip]
c--------- 1 root   root   5,  2 May 20 06:14 ptmx

This makes tmux, screen unrunnable (they just exit with no error, so this was somewhat hard to track down) and sometimes crashes them if they are already running. No good on a multi-user system!

(If you got here trying to recover your system: sudo chmod a+rw /dev/ptmx)