ECR now supports manifests

[nairb774]

The note at https://github.com/k14s/kbld/blob/ddd2561de7d0a07eaee903767adc30a23c4fb1b8/docs/packaging.md#using-with-aws-ecr might soon be out of date.

According to https://github.com/aws/containers-roadmap/issues/505 manifest support is rolling out with docs being updated: https://docs.aws.amazon.com/AmazonECR/latest/userguide/docker-push-multi-architecture-image.html

[cppforlife]

@nairb774 awesome, thanks for tracking that! ill try it out tomorrow.