[ECS] [request]: Publish region-specific ECR public repos for ecs-agent image

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**Tell us about your request**
Opening this on behalf of a customer (https://github.com/aws/amazon-ecs-agent/issues/2526).
Launching ecs-agent out of Dockerhub relies on Dockerhub availability, for which most ECS users will not have a support relationship. Publish the ECS agent in all of the region-specific ECR repositories in the same way as https://github.com/aws/aws-for-fluent-bit.

**Which service(s) is this request for?**
ECS, ECR

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
AWS has the infrastructure to host the ECS Agent image on per-region ECR repositories. This should be done to remove the requirement that Dockerhub be available to launch new nodes. This is particularly important in an autoscaling scenario. AWS customers have a commercial relationship with AWS so can reasonably expect support for the availability of ECR, not so much with Dockerhub.

https://github.com/aws/aws-for-fluent-bit already does something similar.

**Are you currently working around this issue?**
Relying on Dockerhub

Start by reviewing the request and comparing it with the linked aws-for-fluent-bit approach for region-specific ECR repositories. Done would mean the ECS agent image is published in all region-specific ECR repositories so launching new nodes no longer depends on Dockerhub.

Written by the indexing model from the issue text.