Support for private ECR images

### Before opening, please confirm:

- [X] I have checked to see if my question is addressed in the [FAQ](https://github.com/aws-amplify/amplify-hosting/blob/master/FAQ.md).
- [X] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-hosting/issues?q=is%3Aissue+).
- [X] I have read the guide for [submitting questions](https://github.com/aws-amplify/amplify-hosting/blob/master/CONTRIBUTING.md).
- [X] I have removed any sensitive information from my code snippets and submission.

### App Id

NA

### AWS Region

us-east-1

### Amplify Hosting feature

Custom build image

### Is your feature request related to a problem? Please describe:

NA

### Describe how you'd like this feature to work

To use private ECR image as custom build image for amplify application.

The issue names no files, tests, or entry points. Start by locating the custom build image handling in the repository and trace how image access is configured; done means an Amplify application can use a private ECR image with the required access behavior documented or verified.

Written by the indexing model from the issue text.