The title says it all. There are really old questions that ask the same thing, but I think rails has moved on since then.
In my Gemfile, I've tried:
gem 'rails', github: 'rails/rails'
But that gives me 6.1.0.rc1 be11d1b once I run bundle install. Current version of rails as of this writing is 6.1.3. Also tried bundle update rails after using the above line in the Gemfile. So it seems that a version older than current is being used when I try to pull from GitHub...?