[ECR] [request]: unauthenticated access to GHCR

### 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**
I want to be able to configure an ECR pull-through cache rule for public GitHub Container Registries (GHCR) without having to provide credentials.

**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?**
At the moment I can connect to a GHCR but I have to provide credentials. The only images I want to get are in public repos and so I do not want to have to manage tokens (which will expire and/or are linked to individuals).

**Are you currently working around this issue?**
I am currently avoiding the ECR entirely and pulling them from ghcr.io directly. This misses out on the chance to do image scanning etc.

No files, tests, or code entry points are named. Start by reviewing ECR pull-through cache rules and the three issue comments, then define done as configuring a public GHCR source without credentials while preserving the stated ECR image-scanning use case.

Written by the indexing model from the issue text.