[ECR] [request]: Pull through cache support for Elastic Container Registry (docker.elastic.co)

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**Tell us about your request**
We would like to use pull through cache with elastic public registries.

**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?**
Would like to add a pull-through cache rule for [docker.elastic.co](http://docker.elastic.co/) public registries to allow a single source for container images (ECR).

**Are you currently working around this issue?**
We are currently unable to use pull through cache with docker.elastic.co registries as it is not yet supported as an upstream source registry.

**Additional context**
The Elastic Registry is public and does not require credentials.

**Attachments**
https://www.docker.elastic.co/

No repository files or tests are identified. Start by reviewing the ECR pull-through cache documentation and the supported upstream registries, then assess the requirements for docker.elastic.co. Done would mean ECR supports docker.elastic.co as an upstream source for pull-through cache rules.

Written by the indexing model from the issue text.