[ECS] [request]: Proxy Settings to container automatically

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**Tell us about your request**
What do you want us to build?
Make it possible to automatically pass proxy settings to containers as described in docker documentation (using docker 19.03.1) [here](https://docs.docker.com/network/proxy/#configure-the-docker-client):

` In Docker 17.07 and higher, you can configure the Docker client to pass proxy information to containers automatically.`

**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?**
What outcome are you trying to achieve, ultimately, and why is it hard/impossible to do right now? What is the impact of not having this problem solved? The more details you can provide, the better we'll be able to understand and solve the problem.

Pass proxy settings to container automatically.

**Are you currently working around this issue?**
How are you currently solving this problem?
Currently have to pass environment variables to the container through the task definition.

If the container is started manually (after setting proxy settings in ~/.docker/config.json), the proxy environment variables are set in the container as expected. However, tasks started via ECS, dont get these proxy environment variables populated automatically without passing them through the task definition.

**Additional context**
Anything else we should know?

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

Start by reviewing the ECS task-definition behavior and the Docker proxy configuration documentation linked in the issue. Determine how proxy settings from ~/.docker/config.json could reach ECS-launched containers without explicit task-definition environment variables. Done means the behavior is supported and verified for ECS tasks, with documentation or tests covering it.

Written by the indexing model from the issue text.