Docker Registry has the in-built capability of sending notifications based on registry activities:

Notifications are sent to the endpoints via HTTP. This complete notification is based on the listener and broadcaster architecture. Each endpoint has its own queue and all actions (push/pull/delete) trigger the events. These events are queued and once the event reaches the end of the queue, it triggers an HTTP request to the endpoint. The events are sent to each endpoint, but the order is not guaranteed.

Events have a well-defined JSON structure and are sent as the body of the notification. One or more events are sent in the structure and are called an envelope. An envelope may contain one or more events. The registry is also capable of receiving responses from endpoints. The responses with 2XX or 3XX response codes are considered as valid responses and consider the message delivered.