more humane tag format

more humane tag format

diff --git a/Makefile b/Makefile
index 2f46946..c225140 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 IMAGE := discourse/logstash
-TAG := $(shell grep FROM Dockerfile | cut -d: -f2)-$(shell date +%s)
+TAG := $(shell grep FROM Dockerfile | cut -d: -f2)-$(shell date -u +%Y%m%d.%H%M%S)
 
 .PHONY: push
 push: build

GitHub sha: 281a04744bf20b08f8146c11e90271e8d7b81de2