[ECS/Fargate] [request]: ECS Exec : support readonlyRootFilesystem containers

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**Tell us about your request**

I would like to use the ECS Exec feature with readonlyRootFilesystem enabled containers.

**Which service(s) is this request for?**

ECS/Fargate

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

Currently readonlyRootFilesystem enabled containers are not supported, the AWS managed agent crash soon after launch.

**Are you currently working around this issue?**

Yes. I've managed to get it working with `readonlyRootFilesystem: true` by mounting `/managed-agents`, `/var/lib/amazon/ssm` and `/var/log/amazon/ssm` as writable volumes inside.

**Additional context**

https://github.com/aws-containers/amazon-ecs-exec-checker/issues/21

The issue names no repository files or tests. Start with the linked amazon-ecs-exec-checker issue and the reported readonlyRootFilesystem workaround; done would mean ECS Exec works with readonly root filesystems without requiring those writable volume mounts.

Written by the indexing model from the issue text.