Clarify image downloading in k8s docs

Close #362

docs/configuration.rst

@@ -186,6 +186,8 @@ To renew the certificate, run this command once per month::
 
 .. _customise:
 
+.. _custom_openedx_docker_image:
+
 Custom Open edX docker image
 ----------------------------
 
@@ -241,6 +243,8 @@ Then you must rebuild the openedx Docker image::
 
 Finally, you should enable your theme with the :ref:`settheme command <settheme>`.
 
+.. _custom_extra_xblocks:
+
 Installing extra xblocks and requirements
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

docs/k8s.rst

@@ -147,6 +147,15 @@ All non-persisting data will be deleted, and then re-created.
 Guides
 ------
 
+Running a custom "openedx" Docker image
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Some Tutor plugins and customization procedures require that the "openedx" image be rebuilt (see :ref:`customization <custom_openedx_docker_image>`). This is for instance the case if you want to :ref:`install a custom XBlock <custom_extra_xblocks>` or :ref:`run an edx-platform fork <edx_platform_fork>`. When running Open edX on Kubernetes, your custom images will have to be downloaded from a custom registry. You should define a custom image name, build the image and then push them to your custom registry. For instance, for the "openedx" image::
+
+    tutor config save --set "DOCKER_IMAGE_OPENEDX=docker.io/myusername/openedx:{{ TUTOR_VERSION }}"
+    tutor images build openedx
+    tutor images push openedx
+
 Updating docker images
 ~~~~~~~~~~~~~~~~~~~~~~