Devcontainer for Visual Studio Code

[technicalpickles]

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Over in https://github.com/thoughtbot/shoulda-matchers/pull/1485 I was having a hard time reproducing the issue locally on macOS. In trying to find a way to reproduce it, I was remidned of VScode's devcontainer: https://code.visualstudio.com/docs/remote/containers

I generated this with "Remote-Containers: Add Development Container Configuration Files", and chose Ruby on Rails with PostgreSQL and used Ruby 2.7. RVM is installed, so it's easy enough to install/use 2.6 or 3.0.