Watch progress of docker image pull when task is in PENDING state

### Summary
Often times when I have brand new ECS instances being spawned in my cluster and perform a first-pull of the base image plus the additional layers on my ECR repo it takes quite a while for the task to transition from PENDING to RUNNING state.

### Description
This is more of a feature request (or perhaps an RFC to learn how to perform this some other way), but right now what I have to resort to is remotely connect to the ECS instance that is pulling the image defined in the task definition and run a parallel `docker pull` which thanks to recent improvements in the engine allows me to watch the progress of the original docker pull issued by the ECS agent.

If not directly on the ECS console, shown as log or %, at least something that the ECS agent may persist in its log and eventually ship to CloudWatch Logs/Metrics so it's easier to keep an eye out for the progress of the pull instead of just waiting in there.

I suppose failures to launch could be reported that way also since I currently have to dig thru the ECS agent logs to figure out more details aside from the little displayed on the console.

Start with the ECS agent logs and the ECS console's PENDING task view, then compare the available Docker pull and ECR events with CloudWatch Logs/Metrics. Done would be a defined, supported way to expose image-pull progress or launch failures while a task remains PENDING, with the reporting surface and acceptance criteria agreed.

Written by the indexing model from the issue text.