diff --git a/Makefile b/Makefile
index dddd8c9..a457be8 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 #!/usr/bin/make -f
 
 SHELL := /bin/sh
-.SHELLFLAGS = -eu -c
+.SHELLFLAGS := -eu -c
 
 DOCKER := $(shell command -v docker 2>/dev/null)
 GIT := $(shell command -v git 2>/dev/null)
@@ -20,9 +20,10 @@ endif
 IMAGE_LATEST_TAG := $(IMAGE_NAMESPACE)/$(IMAGE_NAME):latest
 IMAGE_VERSION_TAG := $(IMAGE_NAMESPACE)/$(IMAGE_NAME):$(IMAGE_VERSION)
 
-IMAGE_DOCKERFILE := ./Dockerfile
 IMAGE_TARBALL := $(DISTDIR)/$(IMAGE_NAME).tgz
 
+DOCKERFILE := ./Dockerfile
+
 ##################################################
 ## "all" target
 ##################################################
@@ -37,9 +38,9 @@ all: save-image
 .PHONY: build-image
 build-image:
 	'$(DOCKER)' build \
-		--tag '$(IMAGE_LATEST_TAG)' \
 		--tag '$(IMAGE_VERSION_TAG)' \
-		--file '$(IMAGE_DOCKERFILE)' ./
+		--tag '$(IMAGE_LATEST_TAG)' \
+		--file '$(DOCKERFILE)' ./
 
 ##################################################
 ## "save-*" targets