Fargate tasks: ability to set a proxy to be used

### Summary
Feature request for Fargate tasks: ability to set a proxy to be used for pulling Docker images and automatically set the proxy env variables inside containers (as per https://docs.docker.com/network/proxy/#configure-the-docker-client)

### Description
In environments where security is paramount it is very common to have to use a proxy for all outbound Internet traffic. Given AWS ECR currently doesn't offer a VPC endpoint, pulling images from it requires an Internet connection. It's possible to set such a proxy configuration using EC2 ECS tasks, but not when running Fargate tasks (https://docs.aws.amazon.com/AmazonECS/latest/developerguide/AWS_Fargate.html#fargate-tasks-services-networking).
Also of great usefulness would be if this proxy configuration would automatically propagate into launched containers of a Fargate task.

EDIT: There's now support for ECR on PrivateLink: https://aws.amazon.com/about-aws/whats-new/2019/01/aws-fargate--amazon-ecs--and-amazon-ecr-now-have-support-for-aws/, so this issue is only relevant for people wishing to use private repositories other than ECR.

Start by reading the linked AWS Fargate networking documentation and Docker proxy configuration guidance, including the note about ECR PrivateLink. Define how a proxy would be configured for Fargate image pulls and how its variables would propagate into task containers; done should cover both behaviors for private repositories beyond ECR.

Written by the indexing model from the issue text.