Motivation

• Different APIs may benefit from having different base images
• Allow rolling updates when updating the base image

Design

Add a top-level key under predictor image

- name: iris-classifier predictor: type: python path: predictor.py image: ecr/dockerhub link 

• image defaults to cortex default images if user doesn't specify key
• validate that the image path, check access to image (prioritize DockerHub, we may be able to add ECR later)