FYI the reason this generates a different error is that its failing on our URL validator. So receiver.local:5000 passes the validation and so sends a real test request, but isn’t actually a real DNS entry I assume, where-as http://reciever:5000 does not pass the validation and no request is ever set.
For now, one work around would be to setup your containers /etc/hosts so receiver.local maps to receiver, then you can set receiver.local in Baserow and it will validate and allow that URL but get mapped to the real hostname by /etc/hosts.