rebuild.sh: enclose images names in single quotes to simplify debugging.

rebuild.sh

@@ -20,8 +20,8 @@ for dir in $CHANGED; do
     esac
 done
 
-echo "Changed base images: $BASE"
-echo "Changed late images: $LATE"
+echo "Changed base images: '$BASE'"
+echo "Changed late images: '$LATE'"
 
 for b in $BASE; do
     make -C $b push