feat: add docker enviroment for tests

AGENTS.md

@@ -26,6 +26,7 @@ Maintain a deliberately small, self-hosted rota application for a non-profit bar
 - SQLite by default
 - No frontend build step or JavaScript framework
 - Docker Compose for self-hosting
+- Docker Compose `test` profile for running tests independently of the host Python environment
 
 ## Important files
 
@@ -57,6 +58,16 @@ python -m compileall -q app
 pytest -q
 ```
 
+Prefer the containerized suite when the host Python environment is uncertain:
+
+```bash
+make test-docker
+# or
+docker compose --profile test run --rm --build tests
+```
+
+The `test` target of the `Dockerfile` installs `requirements-dev.txt` on top of the app image and runs `pytest` with throwaway settings (`SECRET_KEY`, `DATABASE_URL`, `INITIAL_ADMIN_PASSWORD`). It mounts no host volumes and does not read `.env`, so it is safe on a fresh clone.
+
 For route/UI changes also start the app and verify `/health`, `/login`, the rota, admin month editor, and swap workflow.
 
 ## Security notes