Add Multiple SCMs Plugin

Many freestyle jobs are using Multiple SCMs (multiple-scms) Plugin.

Change-Id: I27bccb214391b8408cc040f0d8a6260441530c29
diff --git a/Dockerfile b/Dockerfile
index 68a5104..4843979 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -97,6 +97,7 @@
         lockable-resources \
         matrix-auth \
         monitoring \
+        multiple-scms \
         permissive-script-security \
         pipeline-utility-steps \
         prometheus \