Hello @jackm-ts
The feature is released, you should be able to build an image from a base image belonging to a private container registry.
To do so, you need to create a registry in your organization with a URL that match the one you set in your dockerfile with the FROM xxxx directive.
After you have created the registry in your orga, you should be able to deploy as usual. During your build phase, you should see that we log in into this new registry to be able to pull the image.
Let me know if you have some issue, so I can take a look at it.