Docker build based on ECR image

In a Dockerfile we require an ECR prebuild image like:

FROM xxxxxxx.dkr.ecr.eu-west-1.amazonaws.com/my-image:latest

Which requires AWS ECR login. It would be nice if we can have a VSTS task for that.

Start with the Dockerfile example and the requirement for an AWS ECR login before pulling its base image. Define the expected VSTS task behavior and verify that a Docker build can use the referenced ECR image after the task runs.

Written by the indexing model from the issue text.