[ECR] [request]: Provide ability to migrate an ECR to a different account and/or region

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**Tell us about your request**

We'd like to be able to migrate an entire ECR repository between accounts and/or regions in order to allow us to change the layout of our architecture. Without a first class feature for handling this, we'd need to resort to pulling every image in every repository, retagging them all, and re-pushing them all, and that would be error prone, and require a lot of bandwidth.

**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?**

The main problem we're trying to solve is to make sure ECR repositories are placed into the correct accounts within our AWS organisation, and to allow us to move things around when that structure changes, without being locked in to whatever choices we made when the repository was originally created.

Without a first-class feature for migrating a repository to a different account or region, it's quite messy to do this right now.

**Are you currently working around this issue?**

At the moment, repositories have to stay where they were created, and we set up cross-account access with IAM policies, however I believe can result in additional egress charges depending on how the ECR repository is accessed, and can result in cross-region charges depending on which accounts need to access to repository.

We're also exploring pull-through caches to try to mitigate some of this when pulling, but it adds complexity and you'd still need to know where the primary copy resided for pushing.

**Additional context**

N/A

**Attachments**

N/A

No repository files, tests, or implementation entry points are identified. Start by reviewing ECR's current cross-account IAM and pull-through-cache workarounds, then define the scope and completion criteria for first-class repository migration between accounts and regions.

Written by the indexing model from the issue text.