If your application needs to respond to webhooks, you'll need some way to expose localhost to the internet. smee.io is a small service that uses Server-Sent Events to proxy payloads from the webhook source, then transmit them to your locally running application.
Webhook Emitter
localhost
Tell your webhook source to send payloads to your smee.io channel, then either use the smee client or, if you're using Probot to build a GitHub App, just set the environment variable.