`ruby-head` is coming with bundler-2 out of the box, so `bundler: 1` is basically ignored

[ojab]

Installing Bundler
  /home/runner/.rubies/ruby-head/bin/gem install bundler -v ~> 1 --no-document
  Successfully installed bundler-1.17.3
  1 gem installed
  Took   0.72 seconds
bundle install
  /home/runner/.rubies/ruby-head/bin/bundle config --local path vendor/bundle
  /home/runner/.rubies/ruby-head/bin/bundle lock
  Fetching gem metadata from https://rubygems.org/...........
  Fetching gem metadata from https://rubygems.org/.
  Resolving dependencies........................................................................
  Bundler could not find compatible versions for gem "bundler":
    In Gemfile.rails-3.x:
      rails (~> 3.0) was resolved to 3.2.15.rc3, which depends on
        bundler (~> 1.0)
  
    Current Bundler version:
      bundler (2.2.0.rc.2)
  This Gemfile requires a different version of Bundler.
  Perhaps you need to update Bundler by running `gem install bundler`?

[eregon]

This would work fine if you have a checked-in Gemfile.lock which was created with Bundler 1.
Is there a reason you specifically want Bundler 1 yet no Gemfile.lock to actually help that?

It seems very inconvenient for anyone using the project if they have to manually bundle _1_ install/etc.

In any case, I think Rails 3 has no chance at all to work on ruby-head. I think you'll want at least Rails 5 or 6.
Ruby 2.7+ needs Rails 5+ from what I tested (notably kwargs issues and BigDecimal.new removed).