Prototype configuration of a devcontainer to speed up dev-env configuration

[cspotcode]

The concept and usage are explained here: https://github.com/cspotcode/node-code-ide-configs/tree/node-devcontainer/.devcontainer#readme

VSCode lets you describe how to run within a dockerized dev environment. VSCode will mount a host directory into the container and run the editor experience within the container. For example, the editor's integrated terminal runs inside the container.

This configuration leverages that feature to quickly give you:

• a fully-functional node dev environment
• a pre-populated build cache. make -j4 should take about 2mins the first time, not an hour

The image can be built either from scratch or by starting with a previously-built image. The idea is that a CI automation could efficiently keep this image up-to-date with the latest master by rebuilding nightly.

I'm opening this PR to get eyes on the idea and to see if anything similar has been attempted in the past.