kubefirst should leverage a pull through cache on images hosted in Docker Hub

[johndietz]

dockerhub images can be problematic due to their rate limits. ecr has the ability to establish a pull through cache for the dockerhub upstream so we don't depend on docker for anything beyond first-time image pulls. this would also improve our reliability for kubefirst local attempts in a kubecon type environments where folks have to share public ip space.

[mimmus]

If not, 'k8s-image-swapper':
https://estahn.github.io/k8s-image-swapper/
is an easy to use, on-the-fly transparent cache.