Fix undefined variable TAGS with INPUT_TAGS (#54)

diff --git a/start.sh b/start.sh
index e4a0a3d..824d76d 100755
--- a/start.sh
+++ b/start.sh
@@ -18,7 +18,7 @@
     _FORCE_OPTION='--force'
 fi
 
-if ${TAGS}; then
+if ${INPUT_TAGS}; then
     _TAGS='--tags'
 fi