Use libwebp version as docker tag.

Makefile

@@ -1,7 +1,8 @@
-repo = "github.com/harukasan/go-libwebp"
-build_dir = "/tmp"
+repo = github.com/harukasan/go-libwebp
+build_dir = /tmp
 cur_dir = $(shell pwd)
-libwebp_so = "${LIBWEBP_PREFIX}/lib/libwebp.so"
+libwebp_so = ${LIBWEBP_PREFIX}/lib/libwebp.so
+LIBWEBP_VERSION ?= 0.5.1
 
 all: test
 
@@ -12,21 +13,21 @@ libwebp: $(libwebp_so)
 
 $(libwebp_so):
 	cd $(build_dir) \
-	&& wget http://downloads.webmproject.org/releases/webp/libwebp-${LIBWEBP_VERSION}.tar.gz \
-	&& tar xf libwebp-${LIBWEBP_VERSION}.tar.gz \
-	&& cd libwebp-${LIBWEBP_VERSION} \
-	&& ./configure --prefix=${LIBWEBP_PREFIX} \
+	&& wget http://downloads.webmproject.org/releases/webp/libwebp-$(LIBWEBP_VERSION).tar.gz \
+	&& tar xf libwebp-$(LIBWEBP_VERSION).tar.gz \
+	&& cd libwebp-$(LIBWEBP_VERSION) \
+	&& ./configure --prefix=$(LIBWEBP_PREFIX) \
 	&& make \
 	&& make install
 
 docker-test:
-	docker run -v $(cur_dir):/go/src/$(repo) -it go-libwebp
+	docker run -v $(cur_dir):/go/src/$(repo) -it go-libwebp:$(LIBWEBP_VERSION)
 
 docker-sh:
-	docker run -v $(cur_dir):/go/src/$(repo) -it go-libwebp sh
+	docker run -v $(cur_dir):/go/src/$(repo) -it go-libwebp:$(LIBWEBP_VERSION) sh
 
 docker-build:
-	docker build  -t go-libwebp .
+	docker build -t go-libwebp:$(LIBWEBP_VERSION) .
 
 docker-clean:
 	docker rm $$(docker ps -a -q -f "ancestor=go-libwebp")
@@ -39,4 +40,3 @@ docker-clean:
 	docker-sh \
 	docker-build \
 	docker-clean
-