Explain how to fix issues with triggered builds and docker containers

install/docker-ci/README.md

@@ -54,6 +54,16 @@ $ mkdir -p /etc/docker/certs.d/docker.idiap.ch
 $ scp ./docker.crt root@<machine>:/etc/docker/certs.d/docker.idiap.ch/
 $ scp ./gitlab.crt root@<machine>:/etc/docker/certs.d/docker.idiap.ch/
     ```
+    As of today, you'll also need to make sure the `root` account has access to
+    the private docker registry (if you're using one) and you need to trigger
+    builds through the gitlab API. So, login to it:
+    ```
+$ whoami
+root
+$ docker login your.registry.tld
+    ```
+    Keep the root account signed into the registry at all times to fix this
+    issue.
 
 
 ## Kernel Support