[Fargate/ECR] [request]: Support IPv6 endpoint for image pulls

### Community Note

* Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request
* Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request
* If you are interested in working on this issue or have submitted a pull request, please leave a comment

**Tell us about your request**
Fargate currently only pulls images from ECR using the ipv4 endpoint, which makes it impossible to use Fargate without assigning public IPv4 addresses or paying for a privatelink endpoint. Even if you provide the dualstack endpoint for the image in the container definition (e.g. \.dkr-ecr.\.on.aws), the task startup fails after a few minutes with an error like:

> ResourceInitializationError: unable to pull secrets or registry auth: The task cannot pull registry auth from Amazon ECR: There is a connection issue between the task and Amazon ECR. Check your task network configuration. operation error ECR: GetAuthorizationToken, exceeded maximum number of attempts, 3, https response error StatusCode: 0, RequestID: , request send failed, Post "https://api.ecr.REGIONXXX.amazonaws.com/": dial tcp 3.102.50.197:443: i/o timeout

**Which service(s) is this request for?**
Fargate/ECR

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I'm trying to use fargate tasks as the targets for a dualstack-without-public-ipv4 ALB without incurring additional costs for ipv4 addresses, by running the fargate tasks in a subnet that uses ipv6 for outbound communication.

The issue names no repository files, tests, or implementation entry points. Start by reviewing the Fargate/ECR image-pull and GetAuthorizationToken path described in the report, then verify the required IPv6 endpoint behavior for IPv6-only outbound tasks. Done means Fargate can pull ECR images without public IPv4 addresses or a PrivateLink endpoint.

Written by the indexing model from the issue text.