Image detection has hard dependency on gcr.io structure

Hi,

It looks like the registry has a hard dependency on image name to be like gcr.io with a project id (i.e. 2 levels). image_name = "gcr.io/<google-project-id>/<prefix>-name:<tag>"

https://github.com/jupyterhub/binderhub/blob/64bb3c7fb2a501ff31e1f29e572758f2589b5d97/binderhub/builder.py#L273

However, the documentation mentions to set up custom registry without the project ID, which causes the image to be rebuilt all the time. image_name = "your-registry.io/<prefix>-name:<tag>"

I’ve worked around it by just adding a projectID to my image prefix for ACR but was wondering if the correct fix for supporting all custom registries as well as gcr registry is what I’m proposing here: https://github.com/shibbas/binderhub/commit/c4a31cd893cd527c9f0479c6afacca24dca7961e

If not, perhaps an update to the documentation for custom registries?

[sgibson91]

I was going to write up some docs about connecting an Azure Container Registry, as per issue #869, today and include this 🙂

[shibbas]

Yup, i can do that. Thanks for the feedback!