Add support for AWS ECR variant of RegistryAuthSupplier

[56quarters]

Much like there is a RegistryAuthSupplier for authenticating with Google's container repository, there could be an implementation that works with AWS ECR repositories. This implementation could make use of the AWS Java SDK to generate ECR credentials on the fly.

A rough sketch of the API would be...

public class AwsEcrRegistryAuthSupplier implements RegistryAuthSupplier {

    private final AmazonECR client;

    public AwsEcrRegistryAuthSupplier(AmazonECR client) {
        this.client = client;
    }

    @Override
    public RegistryAuth authFor(String imageName) throws DockerException {
        client.getAuthorizationToken(/* ... */)
        return null;
    }
    
    // ... other methods
}

Would you accept a pull request that added support for this using the existing ContainerRegistryAuthSupplier Google implementation as a model?

If this functionality was accepted, a follow up issue + PR would probably add use of it to the spotify/dockerfile-maven plugin.

Thanks!

[davidxia]

See #1013 and #876.

[56quarters]

Whoops, missed those PRs