[ECR] [request]: Pull through cache for GovCloud regions

[joshsizer]

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Tell us about your request
It would be fantastic to be able to utilize ECR pull through cache in AWS GovCloud regions

• AWS GovCloud (US-East) (us-gov-east-1)
• AWS GovCloud (US-West) (us-gov-west-1)

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 would like to create an ECR pull through cache in AWS GovCloud regions, which is currently unavailable.

Are you currently working around this issue?
We do not utilize pull through cache, and instead have automation that creates repositories and pushes images into our ECR registry.

[joshsizer]

Relates to #939