This server was overdue for a migration to new hardware, and I used this opportunity to make its setup reproducible by basing it on Docker containers. This allowed me to test things locally, setting everything up on the real server was only a matter of one hour then. Some issues I didn’t recognize locally however, most importantly Docker’s weird IPv6 support. Everything worked just fine when the server was accessed via IPv4, accessing it via an IPv6 address caused connections to hang however. I hit this issue with Docker 1.13.1 originally, updating to Docker 17.12 didn’t change anything. Figuring this out took me quite a while, so I want to sum up my findings here.