Document using a custom registry

[treeder]

Should include:

• Starting up a registry - https://docs.docker.com/registry/
• Use that registry in image in func.yaml?
• Pushing function to that registry
• Use that registry when creating route

[ptone]

This is more than just a documentation issue no?

Looking at the FIXME at the bottom of

https://github.com/iron-io/functions/blob/master/api/runner/task.go

It seems like there has been an admirable effort to avoid a giant bag of global configuration - but this is a case where you would need something analogous to the gitconfig byHost entries - so that from a fully qualified image path - you could reference globally configured credentials per registry.

Is the stop-gap to have a AppCreateListener watch for route additions and pre-fetch the image using a custom docker client?

[ptone]

Also will be good to consider how credentials can be registered as hooks at runtime so that registries using short term credentials can be dynamic:

https://cloud.google.com/container-registry/docs/advanced-authentication