Welcome home, or away. 
So I’d fix up ~/src/discourse_docker/image/discourse_dev/postgres_dev.template.yml and submit the PR and then after you accepted the PR and rebuilt the image, I could test whether it really worked, right?
If that’s the process, then I think I could contrive to modify ~/src/discourse/bin/docker/boot_dev to pull my version of that image to test it.
I’ve got three imports running right now. I think that I may end up just spinning up Digital Ocean droplets to run them, but this could be one way to manage running multiple imports at once on a single machine. Or maybe I should be cranking up a VM in VirtualBox. . .