Added github plugin

Change-Id: I145e7b53597fef731a5be30c6f7abb0ec13525eb
diff --git a/Dockerfile b/Dockerfile
index 424b01b..62c43bb 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -88,6 +88,7 @@
         extensible-choice-parameter \
         gerrit-trigger \
         git \
+        github \
         heavy-job \
         ldap \
         lockable-resources \