tests/docker: remove the Debian base images

We no longer use these in any of our images. Clean-up the remaining
comments and documentation that reference them and remove from the
build.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220914155950.804707-31-alex.bennee@linaro.org>
diff --git a/docs/devel/testing.rst b/docs/devel/testing.rst
index f35f117..aea5b42 100644
--- a/docs/devel/testing.rst
+++ b/docs/devel/testing.rst
@@ -375,7 +375,7 @@
 
 .. code::
 
-   make docker-image-debian10 NOCACHE=1
+   make docker-image-debian-arm64-cross NOCACHE=1
 
 Images
 ~~~~~~