Have you considered ngrok? You can open up a private tunnel from your local environment (local machine, container, device, whatever) to the public internet in seconds. Then you can layer on IP restrictions and even webhook verification to block any traffic not from your provider of choice. It's as simple as:

ngrok http 80 --verify-webhook=slack --verify-webhook-secret=[secret]

with a ton of providers out of the box: https://ngrok.com/docs/cloud-edge#webhook-verification

Also, we recently launched https://webhooks.fyi/ to serve as a community resource to capture patterns & practices around webhook implementations. That's a github pages site so pull requests welcome!

Disclosure: I work at ngrok and helped create webhooks.fyi :)