Tag downloaded nginx image with :candidate before xos buildImages

Change-Id: I50f13bbf0bbe783660e77fc1b509e31a4b6f0acc
(cherry picked from commit 619b635fbb9e57f79cb2e602c041c0a9346831f7)
diff --git a/build.gradle b/build.gradle
index b675cdc..1be5edd 100644
--- a/build.gradle
+++ b/build.gradle
@@ -300,6 +300,7 @@
 project(':onos-apps').deploy.dependsOn project(':maas').deployBase
 
 project(':orchestration:xos').buildImages.dependsOn rootProject."tagxosproject/xos-base"
+project(':orchestration:xos').buildImages.dependsOn rootProject."tagnginx"
 
 project(':platform-install').buildImages.dependsOn project(':orchestration:xos').buildImages