[Fargate] [request]: ECS Fargate (Spot?) no longer allows for file system changes inside container

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**Tell us about your request**
Has the behavior of Fargate 1.4 changed regarding the ephemeral Docker filesystem?

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Prior to ~Aug. 5 2021 we were able to ECS exec inside of our application and perform installation for troubleshooting or convenience similar to the installation of `vim` described here - https://aws.amazon.com/blogs/containers/connecting-to-an-interactive-shell-on-your-containers-running-in-aws-fargate-using-aws-copilot/

We noticed this change in behavior along with failures in our application as it needed to access /tmp and other locations within the file system to write ephemeral files. In order to resolve those endpoints we have gone with explicit Dockerfile "VOLUME" definitions but don't have a great solution to facilitate that for the entire container filesystem. We aren't aware of any changes on our side and even witnessed tasks that have been continuously running since prior to August 5th where the ability to make changes within their file system just randomly stopped.

**Are you currently working around this issue?**
Defining Docker volumes for folders that are changed.

**Additional context**
Primary PID run as root.
Forcing "user: root" in TaskDefinition made no impact
Readonlyrootfilesystem was "null" setting to "false" made no difference.
us-east-1 Region

No repository file, test, or implementation entry point is named. Start by reviewing the reported Fargate 1.4 behavior, the linked ECS Exec example, and the Dockerfile VOLUME workaround; done would require a maintainer-confirmed explanation or roadmap change for ephemeral filesystem writes.

Written by the indexing model from the issue text.