Document the China registry mirror

Kolla always fails to pull kolla images. This patch is to document
how to use the China registry mirror to avoid the failure of pulling
kolla images.

Change-Id: Ib31b3ee6183c12467df4f28f8dff438ed6e9d41c

doc/source/user/troubleshooting.rst

@@ -10,6 +10,24 @@ Failures
 If Kolla fails, often it is caused by a CTRL-C during the deployment
 process or a problem in the ``globals.yml`` configuration.
 
+.. note::
+
+   In some countries like China, Kolla might fail due to unable to pull images
+   from `Docker Hub <https://hub.docker.com/u/kolla/>`__. There is a workround
+   to solve this issue:
+
+   .. code-block:: console
+
+      mkdir -p /etc/docker
+      tee /etc/docker/daemon.json <<-'EOF'
+      {
+        "registry-mirrors": ["https://registry.docker-cn.com"]
+      }
+      EOF
+      systemctl restart docker
+
+   .. end
+
 To correct the problem where Operators have a misconfigured environment,
 the Kolla community has added a precheck feature which ensures the
 deployment targets are in a state where Kolla may deploy to them. To