[ECS] [request]: Support for shell escape in envfile

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**Tell us about your request**
What do you want us to build?
Please add support for shell escapes in envfile and conform to docker-compose env spec

**Which service(s) is this request for?**
This could be 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.

Ability to add/include public/private keys inside envfile.

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

I am currently including them in ecs task definition.

**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.)

Start by reviewing the requested Docker Compose env-file specification and how ECS task definitions currently provide environment values. Define the expected shell-escape behavior for public and private keys, then verify that ECS/Fargate envfiles conform to that specification.

Written by the indexing model from the issue text.