[ECS] [request]: dependencies between tasks

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**Tell us about your request**
What do you want us to build? I would like to have dependencies between tasks within the same service, for example if I have task a that is dependent on task b within the same cluster/service/container then I would like that ability.

**Which service(s) is this request for?**
Fargate, ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
What outcome are you trying to achieve, ultimately, and why is it hard/impossible to do right now? What is the impact of not having this problem solved? The more details you can provide, the better we'll be able to understand and solve the problem.

Dependencies amongst tasks.

**Are you currently working around this issue?**
How are you currently solving this problem?

Right now I have to run steps manually after the tasks come up or I have to build in waits in entry scripts.

**Additional context**
Anything else we should know?

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

The request concerns dependencies between tasks within the same ECS/Fargate service, with manual follow-up steps and entry-script waits as the current workaround. No implementation files or tests are named; first clarify the dependency semantics and lifecycle expectations, and consider the work done when an agreed ECS/Fargate capability replaces those workarounds.

Written by the indexing model from the issue text.