Tag docker images with commit hash, for now

Makefile

+commit := $(shell git rev-parse --short HEAD)
+
 build: dist/go-httpbin
 
 dist/go-httpbin: assets cmd/go-httpbin/*.go httpbin/*.go
@@ -28,7 +30,7 @@ image: assets cmd/go-httpbin/*.go httpbin/*.go
 	mkdir -p /tmp/go-httpbin-docker
 	cp Dockerfile /tmp/go-httpbin-docker
 	GOOS=linux GOARCH=amd64 go build -o /tmp/go-httpbin-docker/go-httpbin ./cmd/go-httpbin
-	docker build -t mccutchen/go-httpbin /tmp/go-httpbin-docker
+	docker build -t mccutchen/go-httpbin:$(commit) /tmp/go-httpbin-docker
 
 imagepush: image
-	docker push mccutchen/go-httpbin
+	docker push mccutchen/go-httpbin:$(commit)