Multiple containers per template / Override task definition in pipeline

### What feature do you want to see added?

I have a scenario where I need a job to deploy two containers simultaneously in my ECS cluster and then run some scripts. Initially, I considered using a pre-created task definition and inheriting its settings. However, I realized that its attributes (such as CPU, memory, environment variables, etc.) don't change when overriding them in the pipeline.

Next, I thought about using templates, which can be overridden during the pipeline execution. However, to the best of my knowledge, only one container can be specified in the template, also I'm unsure if the image can be pulled from ECR, which is the method I use in my task definition.

It would be really helpful if one of these missing features could be implemented!

If anyone has encountered a similar case, I would greatly appreciate any workaround suggestions :)

### Upstream changes

_No response_

### Are you interested in contributing this feature?

_No response_

The issue names no files, tests, or entry points; begin by tracing how the amazon-ecs-plugin represents ECS templates and applies pipeline overrides. Clarify whether the intended result is multiple containers, overridable task-definition attributes, ECR image support, or a combination, then define completion around the selected behavior and its tests.

Written by the indexing model from the issue text.