Support Private Registries for images

[kohidave]

Right now, customers who are using the location field in the manifest have no way to specify credentials. This forces them to use a workaround that involves storing images in ECR.

We should support the image credential field in the container definition.

https://docs.aws.amazon.com/AmazonECS/latest/developerguide/private-auth.html

It'd be nice if we could also just prompt you for the secrets.

[huanjani]

Related: #2061

[kohidave]

Related: https://stackoverflow.com/questions/66803450/authenticate-aws-copilot-app-with-private-docker-hub-repo

[efekarakus]

Related #1177 (comment)