Improve contributing guide (#1130)

Latest MariaDB and shut down command.

Author: adamchainz

CONTRIBUTING.rst

@@ -13,7 +13,7 @@ Run the tests
 
    .. code-block:: console
 
-       docker run --detach --name mariadb -e MYSQL_ROOT_PASSWORD=hunter2 --publish 3306:3306 mariadb:10.8
+      docker run --detach --name mariadb -e MYSQL_ROOT_PASSWORD=hunter2 --publish 3306:3306 mariadb:11.6
 
 3. Run the tests by passing environment variables with your connection parameters.
    For the above Docker command:
@@ -31,3 +31,9 @@ Run the tests
     DB_HOST=127.0.0.1 DB_USER=root DB_PASSWORD='hunter2' tox -e py313-django51 -- tests/testapp/test_cache.py
 
   You can also pass other pytest arguments after the ``--``.
+
+4. When you’re done, shut down the Docker container with:
+
+   .. code-block:: console
+
+      docker rm --force mariadb