Improve Docker build with version information

Change-Id: I76983897cb77730e6c4d659b6aaa92fee54ddc85
diff --git a/Makefile b/Makefile
index debd0f4..1c6225b 100644
--- a/Makefile
+++ b/Makefile
@@ -1,9 +1,14 @@
 MODULE  = github.com/KorAP/Koral-Mapper
 CONFIG  = github.com/KorAP/Koral-Mapper/config
 DEV_DIR      = $(shell pwd)
-BUILDDATE    = $(shell date -u '+%Y-%m-%d_%I:%M:%S%p')
+BUILDDATE    = $(shell date -u '+%Y-%m-%dT%H:%M:%SZ')
 BUILDVERSION = $(shell git describe --tags --abbrev=0 2>/dev/null)
 BUILDCOMMIT  = $(shell git rev-parse --short HEAD)
+GO_LDFLAGS   = -X $(CONFIG).Buildtime=$(BUILDDATE) \
+               -X $(CONFIG).Buildhash=$(BUILDCOMMIT) \
+               -X $(CONFIG).Version=$(BUILDVERSION) \
+               -s \
+               -w
 
 BUILDOUT =
 ifeq ($(ACTION), build)
@@ -12,16 +17,12 @@
 
 
 ifeq ($(strip $(BUILDVERSION)), )
-  BUILDVERSION = "EARLY"
+  BUILDVERSION = EARLY
 endif
 
 build: 
 	go build -v \
-	         -ldflags "-X $(CONFIG).Buildtime=$(BUILDDATE) \
-	                   -X $(CONFIG).Buildhash=$(BUILDCOMMIT) \
-			   -X $(CONFIG).Version=$(BUILDVERSION) \
-	                   -s \
-                       -w" \
+	         -ldflags "$(GO_LDFLAGS)" \
 					--trimpath \
 					$(BUILDOUT) \
 					./cmd/koralmapper/
@@ -43,4 +44,8 @@
 	go test -fuzz=FuzzParseCfgParam -fuzztime=1m ./cmd/koralmapper
 
 docker:
-	docker build -f Dockerfile -t korap/koral-mapper:latest .
+	docker build -f Dockerfile -t korap/koral-mapper:latest \
+		--build-arg BUILDDATE="$(BUILDDATE)" \
+		--build-arg BUILDCOMMIT="$(BUILDCOMMIT)" \
+		--build-arg BUILDVERSION="$(BUILDVERSION)" \
+		.