I have the same issue. First, I thought that my own nginx has to be started a while after the docker containers (yet to confirm), but I found out then that the issue is related to the sock file.
From what I found on stackoverflow 
this issue seems to be a nginx one that is supposed to delete the sock file on termination. The discourse container has nginx v1.7.x Maybe an update to v1.8.x + changing the init.d script would do the trick?
Have a look: