chore(server): change default host binding to 0.0.0.0

Update the default host from localhost to 0.0.0.0 to allow the server
to accept connections from external interfaces when no HOST
environment variable is provided.

main.go

@@ -369,7 +369,7 @@ func main() {
 
 	host := os.Getenv("HOST")
 	if host == "" {
-		host = "localhost"
+		host = "0.0.0.0"
 	}
 	port := os.Getenv("PORT")
 	if port == "" {