I've come across the same issue. FusionAuth and my webhook endpoint are both on the same network (docker compose). I've tried setting the webhook URL to the container name (invoicing.api) with the corresponding port, https://invoicing.api:5001. The call never reaches my api. Testing through Postman or Swagger which are running on the same machine but outside the docker network works fine using https://localhost:5001/fusionauthwebhook.
I came across a similar issue with FusionAuth and my web app authority URL not working when using the container name in the URL, but this was solved by using the local machine IP in it's place. That doesn't seem to work for webhooks. Using the local machine IP in the webhook URL, i.e. https://192.168.0.110:5001/fusionauthwebhook, didn't solve it.