Create bundler setup file. Similar what `bundle --standalone` does.

create_bundler_setup.rb

require "bundler"

class BundlerSetup
  def initialize(gemfile)
    @gemfile = gemfile
    @lockfile = "#{gemfile}.lock"
  end

  def run(io)
    Dir.chdir(File.dirname(@gemfile)) do
      puts_header(io)
      puts_pathes(io)
    end
  end

  private

  def puts_header(io)
    io.puts <<~END
      # DO NOT MODIFY
      #
      # This file has been created by #{$0} at #{Time.now}
      # Run `ruby #{$0}` to regenerated this file

      home = Dir.home
      pwd = File.expand_path("..", __dir__)
      puts "HOME: \#{home}"
      puts "PWD: \#{pwd}"

    END
  end

  def puts_pathes(io)
    defintion = Bundler::Definition.build(@gemfile, @lockfile, {})
    defintion.specs.each do |spec|
      next if spec.name == "bundler"
      spec.require_paths.each do |path|
        path = gem_path(path, spec)
        puts_path(io, path)
      end
    end
  end

  def puts_path(io, path)
    io.puts <<~END
      $:.unshift "#{path}"
    END
  end

  def normalize(path)
    root_dir = File.expand_path("..", Dir.pwd)

    case
    when path.start_with?(Dir.home)
      path.sub(Dir.home, '#{home}')
    when path.start_with?(root_dir)
      relative = Pathname.new(path).relative_path_from(Pathname.new(Dir.pwd))
      "\#{pwd}/#{relative}"
    else
      raise "unhandled path: #{path}"
    end
  end

  def gem_path(path, spec)
    full_path = Pathname.new(path).absolute? ? path : File.join(spec.full_gem_path, path)
    normalize(full_path)
  rescue TypeError
    error_message = "#{spec.name} #{spec.version} has an invalid gemspec"
    raise Gem::InvalidSpecificationException.new(error_message)
  end
end

gemfile = File.expand_path("../Gemfile", __dir__)
setup = File.expand_path("setup.rb", __dir__)

File.open(setup, "w") do |file|
  BundlerSetup.new(gemfile).run(file)
end