testing_sanity.rst: add argument for running in docker (#71223)

docs/docsite/rst/dev_guide/testing_sanity.rst

@@ -22,6 +22,10 @@ How to run
    To run sanity tests using docker, always use the default docker image
    by passing the ``--docker`` or ``--docker default`` argument.
 
+.. note::
+   When using docker and the ``--base-branch`` argument,
+   also use the ``--docker-keep-git`` argument to avoid git related errors.
+
 .. code:: shell
 
    source hacking/env-setup