I just released a new discourse docker base image, I recommend you all update to it because it includes a rather important change that ensures logfiles cycle correctly.
To update run:
cd /var/docker
git pull
./launcher rebuild app
Latest includes a special fix that means that in future, when you need to rebuild:
cd /var/docker
./launcher rebuild app
You get to omit the git pull. 
Of course I am going to have to wait a year for everyone to get on latest… but hey… it is progress