Conflicting Ruby version managers

[parkerbxyz]

Both RVM and rbenv are both installed in the universal image and other images. These version managers conflict with each other and cause problems. I've struggled to use these images in a project with a .ruby-version file because of this conflict.

See also:

• https://stackoverflow.com/a/35808396
• https://github.com/microsoft/vscode-dev-containers/issues/690
• https://github.com/microsoft/vscode-dev-containers/issues/704

[samruddhikhandale]

Hi 👋

This ENV based to disable rbenv / rvm based on an environment variable would also apply to the codespaces-linux image.

Unless things have changed now, as proposed in https://github.com/microsoft/vscode-dev-containers/issues/704#issuecomment-754364204, we should add this support to the images. // cc @Chuxel

We should update the Ruby Feature, and add two new Feature options which controls rvm and rbenv installation. (eg: installRVM, installRbenv)