Honor a configurable port in the reusable Docker health check

[felladrin]

Currently, the reusable Docker workflow hardcodes 7860 in both the readiness curl and the PLAYWRIGHT_BASE_URL, and the compose production ports mapping publishes to container port 7860 while the app actually binds PORT. So a non-default port silently breaks both sides: the health check polls a port nothing answers on and never sees the container ready, and the published mapping points at a dead container port. This PR does 2 things: the reusable workflow now takes a port input (default "7860") exposed as a job-level PORT, so docker compose up, the readiness curl, and the Playwright base URL all agree on the same port; and the compose ports mapping forwards the host port to the container's real port (${PORT:-7860}:${PORT:-7860}). With no input passed the behavior is identical to before (7860:7860), so this is purely a robustness fix. #2276 already fixed the same hardcode in the Dockerfile healthcheck; this closes the CI side.

• Fixes CI Docker health check hardcodes port 7860 instead of using PORT env var #2286.

Note: a job-level env: is needed rather than a per-step one, because docker compose up -d reads PORT from the environment too. And caller-side env: doesn't cross the workflow_call boundary, which is why the port arrives as a declared input.

How to test

1. With PORT unset, run docker compose -f docker-compose.production.yml up -d and check that curl localhost:7860/status returns 200.
2. Run it again with PORT=9000 docker compose -f docker-compose.production.yml up -d and check that curl localhost:9000/status returns 200 (this failed before the fix, since the old mapping pointed at container port 7860).
3. Confirm the check-docker workflow passes on this branch, which covers the default-port path end-to-end.

Note: I couldn't run the Docker build locally in this session, so step 3 is what verified the container path (check-docker / Check Docker Container passed in 2m48s). Step 2 still needs a local run to confirm the override case.