Use DOCKER_TAG instead of CACHE_TAG to know which tag is being built.

Change-Id: I36e2b59aefe96fa1868a24f7440e915fd9da0f25
diff --git a/docker/hooks/build b/docker/hooks/build
index c1d2143..4dff446 100755
--- a/docker/hooks/build
+++ b/docker/hooks/build
@@ -3,12 +3,12 @@
 echo "=> Using custom build hook to add build arguments"
 echo "=> set DEBUG=true to see debug information"
 
+TAG=${DOCKER_TAG:-latest}
+
 if [ "$DEBUG" == "true" ]; then
   env | sort
 fi
 
-TAG=${CACHE_TAG:-latest}
-
 cd /src/${BUILD_CODE}${BUILD_PATH}
 
 if [ "$DEBUG" == "true" ]; then