[ECS] [request]: Proper documentation of ECS / Container Internal Endpoints For Proxy Whitelisting

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**Tell us about your request**
Documentation regarding the endpoints the ECS agent needs to be able to chat with in order to function correctly.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
For our application we're required to do egress filtering on anything inside a CDE environment, it would be good if AWS could keep an up-to-date list of the endpoints that the ECS agent needs to be able to hit in order to function correctly per-region.

**Are you currently working around this issue?**
Viewing the rejections the proxy is filtering and tracing them back to the ECS agent in question and manually whitelisting them.

**Additional context**
Some of the various endpoints i've found thus far for us-west-1 and us-east-1
ecs.us-east-1.amazonaws.com
ecs-a-21.us-east-1.amazonaws.com
ecs-t-21.us-east-1.amazonaws.com
amazonlinux.us-east-1.amazonaws.com
secretsmanager.us-east-1.amazonaws.com
ecs.us-east-2.amazonaws.com
amazonlinux.us-east-2.amazonaws.com
secretsmanager.us-east-2.amazonaws.com
ecs.us-west-1.amazonaws.com
api.ecr.us-west-1.amazonaws.com
*.dkr.ecr.us-west-1.amazonaws.com
ecs-a-1.us-west-1.amazonaws.com
ecs-t-1.us-west-1.amazonaws.com
amazonlinux.us-west-1.amazonaws.com
secretsmanager.us-west-1.amazonaws.com
ecs.us-west-2.amazonaws.com
amazonlinux.us-west-2.amazonaws.com
ec2.us-east-1.amazonaws.com
ec2.us-east-2.amazonaws.com
ec2.us-west-1.amazonaws.com

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

Start by reviewing the endpoint list in the issue and the current ECS, ECR, EC2, Amazon Linux, and Secrets Manager documentation for the regions mentioned. Document the ECS agent's required per-region endpoints and proxy-whitelisting guidance, then verify that the result covers the listed services and endpoint patterns.

Written by the indexing model from the issue text.