[ECS] [Volumes]: Ability to create config "volume" and mount it into container as file

**Tell us about your request**
Would be very nice to be able to mount strings (secrets/configurations) defined in the task definition into the container as a file.

**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?**
Some apps need to be configured via config file on the file system and in order to put it there, we need to build/store a custom image. This is very uncomfortable, especially in the case when the same container can be reused with just as small configuration difference. Being able to define a config file as a "volume" and mount into the container would solve the issue, actually, k8s can do that.

**Additional context**
This could be used for secrets as well (#8).
Kubernetes does it via [ConfigMaps](https://kubernetes.io/docs/concepts/storage/volumes/#configmap)