AWS ECR push image re-tagging

Amazon ECR push works effectively to push built docker images up to AWS ECR even using the task multiple times to give an image multiple tags.

It would be nice to be able to use the same AWS ERC Push task to re-tag a published ECR image by setting the 'Source Image Name' and 'Sounce Image Tag' to be the AWS ECR docker repo with the proper image name and tag..

Start by reviewing the Amazon ECR Push task and its Source Image Name and Source Image Tag inputs. Test the requested flow with an already-published ECR image, and consider the work complete when the task can re-tag that image in ECR without rebuilding or pushing a new image.

Written by the indexing model from the issue text.