Add support for Amazon's EC2 Container Registry (ECR) to Relay

[christophermaier]

Cog users wishing to access container images stored in Amazon's EC2 Container Registry from Relay will run into issues because a docker login yields a token that is effective for only 12 hours. With Relay's current implementation, you'd likely need to resort to things like restart the Relay process on a schedule in order for it to continue working.

Relay should have some smarts to contend with this use case. @kevsmith suggested exposing ECR support as a new engine within Relay.

[christophermaier]

Peter Norton in Cog Slack mentioned https://github.com/awslabs/amazon-ecr-credential-helper. Perhaps adding support for a Docker credentials store could be a way to address this issue, and could be useful beyond just ECR.

[pcn]

This branch implements a limited functionality where only ECR can be used: https://github.com/pcn/go-relay/tree/how-would-this-work. The limitation with that is that any images that would be used need to be loaded into ECR. Requesting images that aren't in the provided ECR registry will result in a referrel to dockerhub being returned, and then trying to get from dockerhub will fail because the ECR creds will be preferred.

[christophermaier]

This issue was moved to operable/go-relay#74