Docker integration lane fails at the Compose test stack step

[arena-ai-coding-agent]

Docker build and integration tests fails on main at the Run isolated Compose test stack step. Both image builds succeed; only the Compose run fails.

Not a regression from #52. The job last passed at 8c4ca5e (2026-08-24) and was skipped on every run afterwards, because it declares needs: [lint, test, lockfiles] and Lockfile reproducibility was red. #52 fixed the upstream jobs, so this lane executed again for the first time in about a week and surfaced the existing breakage.

The only change to its inputs since the last green run is #42, which rewrote backend/requirements.lock. Dockerfile.test, docker-compose.test.yml and backend/pytest.ini are untouched since then.

Already ruled out

• Full suite under the container's exact env (DATABASE_URL -> postgres-test, REDIS_URL, AI_PROVIDER=rule-based, empty Stripe/OpenAI keys): 352 passed, including the 2 integration-marked tests.
• pip install --dry-run -r backend/requirements.lock in a clean venv: resolves cleanly.
• tsc --noEmit clean; vitest run --coverage 48 passed, 95.7% statements.
• Re-ran on a fresh commit — fails identically, so not flaky.

The container command is:

cd backend && pytest && cd ../frontend && npm run typecheck && npm test -- --coverage

Note this runs pytest with no marker filter, so it includes the integration tests, and it runs the frontend suite inside the same container.

Next step: someone with Docker access should run

docker compose -f docker-compose.test.yml up --build --abort-on-container-exit --exit-code-from test-runner

and read the test-runner output. I could not: Docker is unavailable in my sandbox and the Actions log endpoint returns EOF from there, so the specific failing assertion is still unknown.