Introduce address flag (#2)

This can be useful in cirruslabs/cirrus-cli#121
where listening 0.0.0.0 will make the proxy reachable from other
containers that want to use the cache.

cmd/main.go

@@ -9,6 +9,8 @@ import (
 )
 
 func main() {
+	var address string
+	flag.StringVar(&address, "address", "127.0.0.1", "Address to listen on")
 	var port int64
 	flag.Int64Var(&port, "port", 8080, "Port to serve")
 	var bucketName string
@@ -29,7 +31,7 @@ func main() {
 	bucketHandler := client.Bucket(bucketName)
 	storageProxy := http_cache.NewStorageProxy(bucketHandler, defaultPrefix)
 
-	err = storageProxy.Serve(port)
+	err = storageProxy.Serve(address, port)
 	if err != nil {
 		log.Fatalf("Failed to start proxy: %s", err)
 	}

proxy/http_proxy.go

@@ -26,10 +26,10 @@ func (proxy StorageProxy) objectName(name string) string {
 	return proxy.defaultPrefix + name
 }
 
-func (proxy StorageProxy) Serve(port int64) error {
+func (proxy StorageProxy) Serve(address string, port int64) error {
 	http.HandleFunc("/", proxy.handler)
 
-	listener, err := net.Listen("tcp", fmt.Sprintf("127.0.0.1:%d", port))
+	listener, err := net.Listen("tcp", fmt.Sprintf("%s:%d", address, port))
 
 	if err == nil {
 		address := listener.Addr().String()