We’ve got Resin working where the server and device are both on the same air-gapped LAN, so I’m sure pointing to an alternative registry is possible. I’m going to ping the lovely gentleman who did a bunch of the work in creating that setup.
Main reason being that we already have multiple production images running on a private docker registry, so if its possible, it may be a better choice for us to maintain that part of the workflow while leveraging the benefits of Resin. Another reason is purely for enterprise security compliance regarding proprietary codes.
We do have a number of customers who are happy to have completed images stored in our own registries once built (as there are obviously build secrets associated with building these images). This is currently possible on both production and development flavours of resinOS, and the resin CLI tool’s resin deploy (https://github.com/resin-io/resin-cli/blob/master/doc/cli.markdown#deploy-appname-image) allows this functionality to be achieved (you can build the image locally, or use an image pre-existing in your local Docker registry, and then the resulting image is pushed to our registries).