[Fargate/ECS] [Image caching]: provide image caching for Fargate.

EDIT: as @ronkorving mentioned, image caching is available for EC2 backed ECS. I've updated this request to be specifically for Fargate.

**What do you want us to build?**
I've deployed scheduled Fargate tasks and been clobbered with high data transfer fees pulling down the image from ECR. Additionally, configuring a VPC endpoint for ECR is not for the faint of heart. The doc is a bit confusing.

It would be a big improvement if there were a resource (network/host) local to the instance where my containers run which could be used to load my docker images.

**Which service(s) is this request for?**
Fargate and ECR.

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I don't want to be charged for pulling a Docker image every time my scheduled Fargate task runs.
On that note the VPC endpoint doc should be better too.

**Are you currently working around this issue?**
This was for a personal project, I instead deployed an EC2 instance running a cron job, which is not my preference. I would prefer using Docker and the ECS/Fargate ecosystem.