Fix typo for docker-mirror-images job

Change-Id: I812358df860e5b87f1bd6cdfbf3515d95454adfd
diff --git a/docker-mirror-images.groovy b/docker-mirror-images.groovy
index f7a861e..636c666 100644
--- a/docker-mirror-images.groovy
+++ b/docker-mirror-images.groovy
@@ -69,7 +69,7 @@
                         external = true
                         // check if images exists - raise error, as we don't want to rewrite existing one
                         def imageRepo = targetRegistryPath - targetRegistry
-                        if (mcp_artifactory.imageExists(env.REGISTRY_URL, "${imageRepo}/${imageName}", imageTag)) {
+                        if (mcp_artifactory.imageExists(env.REGISTRY_URL, "${imageRepo}/${imageName}", env.IMAGE_TAG)) {
                             error("Image ${targetImageFull} already exists!")
                         }
                     }