Cannot set custom SSL certs or settings

[alexamirante]

Commit 74aea02 broke the possibility of providing custom SSL certs or settings.py as explained in the README here and here.

A workaround is to first start the container, then inspect where the docker volume files are stored on the file system via docker volume inspect docker-zulip_zulip, and finally put the files there. But this wouldn't work if you don't have root privileges on the machine.

[timabbott]

The nature of the breakage wasn't specified in this report, but I think this is probably effectively a duplicate of #343 -- that with named volumes, it's harder to predict the paths to use.