[ECR] [request]: Support Docker Hardened Images (DHI) for pull through cache

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**Tell us about your request**

We request that AWS ECR pull-through cache support Docker Hardened Images from the `dhi.io` registry.

Example image reference: `dhi.io/python:3.10-alpine3.22`

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

We want to use ECR pull-through cache for the `dhi.io` registry to leverage Docker Hardened Images in our AWS container workloads. Currently, ECR pull-through cache supports Docker Hub and other registries, but does not support `dhi.io`.

Start with the issue requirements for AWS ECR pull-through cache and the example reference dhi.io/python:3.10-alpine3.22. Review how ECR currently supports upstream registries; done means pull-through caching works for dhi.io images without changing the requested image reference.

Written by the indexing model from the issue text.