Add a .ruby-version file and specify the ruby version in the Gemfile

[cknoxrun]

For ruby version managers (RVM, rbenv, chruby, etc), a .ruby-version file is used to indicate the version of a ruby a directory should use.

This can simply be the version number, for example: 2.7.1 (which is the latest current release).

Additionally, the Gemfile can contain a ruby version to ensure that when you run bundle it is using the correct version of ruby. For that, just add this to the top of the Gemfile:

ruby '2.7.1'

And re-bundle.