Added email-ext-plugin

Change-Id: I2eb144a4418a2ab8b7a0d1b56f8962f89455796c
diff --git a/Dockerfile b/Dockerfile
index b06a8ea..e0f6692 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -84,6 +84,7 @@
         description-setter \
         discard-old-build \
         docker-workflow \
+        email-ext-plugin \
         extended-choice-parameter \
         extensible-choice-parameter \
         gerrit-trigger \