Allow disabling rbenv and rvm in Ruby containers #704

• VSCode Version: 1.52.1
• Local OS Version: macOS Big Sur 11.1
• Name of Dev Container Definition with Issue: ruby, ruby-rails

Steps to Reproduce:

1. Generate a new Ruby on Rails app
2. Add ruby-rails development container definition
3. Reopen folder in container
4. Open a terminal and cd into the workspace directory (e.g. cd . or cd /workspace from a new Integrated Terminal)
5. Observe that rvm complains about the Ruby version specified in the Gemfile isn't installed, even though the development container specifically has (the correct version of) Ruby installed (see Disable RVM nag warning about a Ruby version not being installed #690).

Why are rbenv and rvm installed on Ruby development containers? We can already select the version of Ruby we want installed by using a particular Docker image tag, so there's no need for a Ruby version manager; they generate unnecessary noise and are difficult to remove from the pre-built containers. At the very least, there should be a mechanism to conditionally prevent loading the rvm and rbenv initialization scripts using an environment variable.