The bit of code that checks the image registry for an existing image is called from:
and the code itself is:
My guess would be that something with checking the registry isn’t quite right. I think there was an issue previously with how the name that is looked up in the registry is formatted or something. The point being that we have had things break around the registry handling before.
The other thing to double check (maybe even first) is if your BinderHub is indeed configured to use a docker registry.