feat: Added instructions for binding locally. (#215)

Added the ability to specify a hostname when executing locally.

The default example in the README now includes a flag to only bind
to the loopback device on a machine so firewall alerts shouldn't
trigger by default (and users won't expose functions to the LAN).

Existing use of the Start function including previous versions and
within the FF Buildpacks for GCF should continue to function.

See FF buildpack template for more details about how it invokes:
https://github.com/GoogleCloudPlatform/buildpacks/blob/main/cmd/go/functions_framework/template_v0.go

README.md

@@ -103,8 +103,16 @@ handling logic.
 		if envPort := os.Getenv("PORT"); envPort != "" {
 			port = envPort
 		}
-		if err := funcframework.Start(port); err != nil {
-			log.Fatalf("funcframework.Start: %v\n", err)
+		
+		// By default, listen on all interfaces. If testing locally, run with 
+		// LOCAL_ONLY=true to avoid triggering firewall warnings and 
+		// exposing the server outside of your own machine.
+		hostname := ""
+		if localOnly := os.Getenv("LOCAL_ONLY"); localOnly == "true" {
+			hostname = "127.0.0.1"
+		} 
+		if err := funcframework.StartHostPort(hostname, port); err != nil {
+			log.Fatalf("funcframework.StartHostPort: %v\n", err)
 		}
 	}
 	```
@@ -113,8 +121,7 @@ handling logic.
 
 1. Start the local development server:
 	```sh
-	export FUNCTION_TARGET=HelloWorld
-	go run cmd/main.go
+	FUNCTION_TARGET=HelloWorld LOCAL_ONLY=true go run cmd/main.go
 	# Output: Serving function: HelloWorld
 	```
 

funcframework/framework.go

@@ -101,11 +101,16 @@ func RegisterCloudEventFunctionContext(ctx context.Context, path string, fn func
 
 // Start serves an HTTP server with registered function(s).
 func Start(port string) error {
+	return StartHostPort("", port)
+}
+
+// StartHostPort serves an HTTP server with registered function(s) on the given host and port.
+func StartHostPort(hostname, port string) error {
 	server, err := initServer()
 	if err != nil {
 		return err
 	}
-	return http.ListenAndServe(":"+port, server)
+	return http.ListenAndServe(fmt.Sprintf("%s:%s", hostname, port), server)
 }
 
 func initServer() (*http.ServeMux, error) {