Add throttle-concurrents plugin

Change-Id: I1d868ca1992b93863648a179b574dc62198efe68
diff --git a/Dockerfile b/Dockerfile
index c26bb84..f928360 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -104,6 +104,7 @@
         slack \
         ssh-agent \
         test-stability \
+        throttle-concurrents \
         timestamper \
         workflow-cps \
         workflow-remote-loader \