[ECS] [request] User Defined Networks

I'm trying to see if [user defined networks ](https://docs.docker.com/v1.11/engine/userguide/networking/dockernetworks/#user-defined-networks) is supported?

I've looked at the task definition options and could not find any place to set the network the container should connect to?

Is it supported yet?

Start with the issue's Docker user-defined networks reference and the ECS task definition options mentioned in the report. Determine whether ECS supports selecting a user-defined network for a container and identify the relevant ECS documentation or feature area. Done means the support status and required task-definition configuration are clearly established or the feature scope is defined.

Written by the indexing model from the issue text.