Add SSH Agent plugin

Change-Id: I4d0e287977db5a0a635533109ace4486057c1fe1
diff --git a/Dockerfile b/Dockerfile
index dc2ba76..c26bb84 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -102,6 +102,7 @@
         rebuild \
         simple-theme-plugin \
         slack \
+        ssh-agent \
         test-stability \
         timestamper \
         workflow-cps \