remove duplicated timestamp

Makefile

@@ -125,7 +125,7 @@ README_HEAD = `cat ./_README.md | sed -e 's/%CONTAINERS_LIST%.*//g'`
 README_TAIL = `cat ./_README.md | sed -e '1,/%CONTAINERS_LIST%/d'`
 
 update_readme:
-	@echo "$(README_HEAD)\n - $(TOOLSET) $(README_TAIL)" | sed -e "s/%LAST_UPDATE%/\n_Last update: `date`_ \n/" > README.md
+	@echo "$(README_HEAD)\n - $(TOOLSET) $(README_TAIL)" | sed -e "0,/RE/s/%LAST_UPDATE%/\n_Last update: `date`_ \n/" > README.md
 
 delete_dangling_images:
 	@echo "Removing dangling images"