[ECR] [request]: Vanity URL for ECR Endpoint

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**Tell us about your request**
Would like to support vanity URLs for ECR instead of the using the AWS ARN user ID followed by the region datacenter URI.

**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.
Like DNS, it is easier for anyone to use a simple domain "containers.company.com" vs. "1234567890ABC.dkr.ecr.us-east-1.amazonaws.com"

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

**Additional context**
#2744
https://repost.aws/questions/QUU4vTb5XDS127j1LOIExacw/using-a-vanity-domain-for-ecr-images
case#176824418300629

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

Start by reviewing issue #2744, the linked re:Post question, and case #176824418300629 to understand the requested ECR endpoint behavior. The issue's stated outcome is allowing a simple domain such as containers.company.com instead of the account-and-region ECR hostname; confirm the request and any documented AWS response before proceeding.

Written by the indexing model from the issue text.