Fixed Dockerfile, dockerized devx, added more doc

[senglin]

Issue #, if available:

1. I've experienced issues following the tutorial to set up new running task or creating new service via fargate cli. The documents mentioned that the AWS SDK is utilized, but did not mention how the .aws/config file can be leveraged. It wasn't clear that a variable has to be set, unless you read through the AWS SDK documentation in detail.
2. There is no instructions for installing the CLI, and also to use the latest code changes if needed.
3. Dockerfile is not building.

Description of changes:

• As best practices, I have added support for devcontaine to support VSCode development container experience.. This would also bring us closer to CodeSpace compatibility once Github releases that capability (feature to spin up development environment within Github).
• Fixed Dockerfile which fails to build.
• Added documentation for contribution as well as integration with AWS
  CLI config file
• Made Dockerfile such that it can be run using Docker run without additional parameters.
• Added documentation for installation using various methods.

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