diff --git a/openshift/README.md b/openshift/README.md
index 3496698f..9a636d92 100644
--- a/openshift/README.md
+++ b/openshift/README.md
@@ -7,7 +7,8 @@ Deploy MBS to OpenShift
 $ docker build openshift/backend \
     --tag mbs-backend:latest \
     --build-arg mbs_rpm=<MBS_RPM> \
-    --build-arg mbs_messaging_umb_rpm=<MBS_MESSAGING_UMB_RPM>
+    --build-arg mbs_messaging_umb_rpm=<MBS_MESSAGING_UMB_RPM> \
+    --build-arg umb_ca_crt=<UMB_CA_CRT>
 ```
 
 where:
@@ -19,6 +20,8 @@ where:
   Plugin](https://github.com/release-engineering/mbs-messaging-umb) RPM. If not
   provided, only `fedmsg` and `in_memory` will be available for messaging in the
   image.
+* UMB_CA_CRT is a path or URL to the CA certificate of the message bus to be
+  used by MBS.
 
 ## Build the container image for MBS frontend