Fix red develop eea3c6e64054

The merge lane found `develop` red at `eea3c6e640542f3ff8fd63c09a52496f93540b88` on its own: no queued PR merges until it is fixed. Failing (tests and gate stages): - `start-the-compose-stack` The whole gate log: artifact `6261` on lane task 7764. Make `develop` pass its full gate again. Open a PR as usual: the merge lane merges a PR that fixes some of these and adds no failure even while `develop` is red.

Develop's code wasn't red: the base check gated a scratch --repo checkout whose seeded .env had no ports, so dc up refused. merge_lane.py now copies the slot's .env into such a tree. Tests pass (117), quick gate green; PR #6174 queued.