Create "kitchen sink" image based on core VS Online image to allow people to build up from it, use it offline

[Chuxel]

Related to #140, #154, and https://github.com/MicrosoftDocs/vsonline/issues/205, we can create a dev container based off of the current VS Online image (which itself is based on Oryx).

This will allow developers to:

1. Simply use it locally if they prefer
2. Make it easier to iterate on new dev containers from it for their own repositories that use it
3. Make it easier to move back-and-forth between VSO and local
4. Improve transparency about its exact contents

//cc: @lostintangent

[Chuxel]

Resolved by #245