Docker registry URLs

The doctl compute image create command requires an --image-url, but I can’t find what that URL looks like. If my registry is named RRR and my image name is going to be NNN, what would the URL be?

The documentation for the command says that a URL is required, but gives no examples of what one would look like. It does frequently mention URLs assigned to running containers, but my understanding is that this image doesn’t even exist yet, let alone run.

Here’s what I have for a command line so far. I’ve already run doctl registry login and querying the registry for info works.

doctl compute image create --image-distribution ubuntu-20-04-x64 --image-url ??? --region nyc1 NNN