[ECR] [request]: ECR automated builds

### Community Note

* Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request
* Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request
* If you are interested in working on this issue or have submitted a pull request, please leave a comment

**Tell us about your request**
Docker Hub provides [auto-build feature](https://docs.docker.com/docker-hub/builds/) for images hosted in its repositories. It can automatically builds images from source code and automatically pushes the built image to Docker repositories. When there's a change in Dockerfile or a base image revision becomes available, the auto-build kicks in and updates the hosted image. Many of the Docker Hub repo owners benefit from it. But currently, this feature is missing in ECR and ECR registry owners have to spend extra time and effort to keep their images up-to-date.

**Which service(s) is this request for?**
ECR

**Are you currently working around this issue?**
We have to build our own workflow to monitor our images' status in ECR and auto-build when needed. But we believe this is a common use case.

The request describes automated builds for images stored in AWS ECR, using Docker Hub auto-builds as the comparison. Start by reviewing the ECR service scope and the stated workflow requirements; done would mean a defined and implemented way to rebuild and push images when source or base-image changes occur.

Written by the indexing model from the issue text.