Update docker docs

docs/docker.rst

@@ -65,7 +65,13 @@ The ``ocrmypdf`` image is also available, but is deprecated and will be removed
 in the future.
 
 OCRmyPDF will use all available CPU cores. See the Docker documentation for
-`adjusting memory and CPU on other platforms <https://docs.docker.com/config/containers/resource_constraints/>`__.
+`adjusting memory and CPU on other platforms <https://docs.docker.com/config/containers/resource_constraints/>`__
+if you are using Docker on macOS or Windows, where you may need to manually assign
+more resources. On Linux, all resources will be available automatically.
+
+The underlying operating system and other details in Docker images are subject
+to change at minor releases. If you are modifying the image, you should pin
+the version you intend to use.
 
 Using the Docker image on the command line
 ==========================================