ECR ignore duplicates: enable the ability to ignore duplicate digest errors on push

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**Tell us about your request**
What do you want us to build?

Better handle duplicate tags on Immutable Registries when digests have not changed.

This could be the addition flag to `--ignoreDuplicateDigests` or handled by default.

**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?**
What outcome are you trying to achieve, ultimately, and why is it hard/impossible to do right now? What is the impact of not having this problem solved? The more details you can provide, the better we'll be able to understand and solve the problem.

Currently when pushing an image to an IMMUTABLE registry, even when the digest has not changed, it fails with `ImageTagAlreadyExistsException` which is helpful when building custom handlers, but when using 3rd party tools for pushing and pulling it causes build errors, when it is not an actual error.

**Are you currently working around this issue?**
How are you currently solving this problem?

We have not yet found a solution, but are thinking of ways to wrap the bazel toolsets we are using in order to swallow the error. But this could have the side effect of swallowing all errors.

**Additional context**
Anything else we should know?

**Attachments**
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Start by reviewing the ECR immutable-registry behavior described here, focusing on pushes that return ImageTagAlreadyExistsException when the digest is unchanged. Compare the proposed --ignoreDuplicateDigests flag with handling duplicates by default, and consider the stated Bazel workaround and its risk of swallowing other errors. Done means duplicate unchanged digests no longer cause third-party image push workflows to fail while genuine errors remain visible.

Written by the indexing model from the issue text.