Use "run --rm" instead of exec for tests in README

* exec requires a running container, "run --rm" starts a new one and deletes when the job is done
* exec requires the related container to stay healthy, "run --rm" run things on a separate container

Author: ulgens

README.rst

@@ -365,8 +365,8 @@ Running Locally with Docker
 
 4. Run the tests::
 
-    docker compose exec web tox
-    docker compose exec web python -m manage test
+    docker compose run --rm web tox
+    docker compose run --rm web python -m manage test
 
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