Add Plot plugin for cvp-spt job

Change-Id: I36231880e074ae263921b9356c5d791196afffa7
diff --git a/Dockerfile b/Dockerfile
index 18d9153..fe97950 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -104,6 +104,7 @@
         performance \
         permissive-script-security \
         pipeline-utility-steps \
+        plot \
         prometheus \
         rebuild \
         simple-theme-plugin \