[ECS] [request]: Run ecs-agent container as a non-root user

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**Tell us about your request**
ECS agent container runs as a non-root user by default.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
The ecs-agent container runs as `root` by default and there is no way to configure it otherwise. Running it as non-root user will have better complicance with seucrity best practices. This is related to #1059 but not exactly the same. This is about the ecs-agent container's running user, not the [Docker daemon's](https://docs.docker.com/engine/security/rootless/).