New DockerHub Image retention policies will delete unused images after 6 months

[scottyhq]

I think only the AWS binderhub is using DockerHub for storage (instead of AWS ECR), but cross posting this issue

Cross-posting pangeo-data/pangeo-docker-images#121

Given the new policies if a binder link isn't used for 6 months, the linked image on DockerHub will be deleted, eventually forcing a rebuild. For binders using a Dockerfile this will simply fail, for binders using repo2docker environment.ymls this will force a rebuild.