Add possibility to use any source image tag

For promote timestamped snapshot to testing

Change-Id: I5d7bcde022b6ac40453cad99de469dbfb83d7ade
Related-PROD: PROD-31298
diff --git a/docker-mirror-images.groovy b/docker-mirror-images.groovy
index dca2462..7ec1092 100644
--- a/docker-mirror-images.groovy
+++ b/docker-mirror-images.groovy
@@ -71,7 +71,7 @@
         try {
             allowedGroups = ['release-engineering']
             releaseTags = ['proposed', 'release',  '2018', '2019', '2020']
-            tags = [env.SOURCE_IMAGE_TAG, env.IMAGE_TAG]
+            tags = [env.IMAGE_TAG]
             tagInRelease = tags.any { tag -> releaseTags.any { tag.contains(it) } }
             if (tagInRelease) {
                 if (!jenkinsUtils.currentUserInGroups(allowedGroups)) {