I tested ractors in ruby with docker and without

this is my script

threads = []
ractors = []
Benchmark.bm do |x|
  x.report("Single thread:") do
    cpu_intensive_task
    cpu_intensive_task
  end

  x.report("Two threads:") do
    2.times do
      threads << Thread.new { cpu_intensive_task }
    end
    threads.each(&:join)
  end

   x.report("Two ractors:") do
     2.times do
       ractors << Ractor.new { cpu_intensive_task }
     end
     ractors.each(&:take)
   end
end

when I run it local I see

       user     system      total        real
Single thread:  0.938003   0.000150   0.938153 (  0.942506)
Two threads:  0.931752   0.000313   0.932065 (  0.937688)
Two ractors:  1.177639   0.000387   1.178026 (  0.592312)

but when I run it docker container I see

       user     system      total        real
Single thread:  1.221073   0.000000   1.221073 (  1.221220)
Two threads:  1.200631   0.001995   1.202626 (  1.201255)
Two ractors:  2.448438   0.000995   2.449433 (  1.314552)

how to see real multithreading in docker?

I tried to configure my container with command like a
docker run --rm -it -v pwd:/app --memory="1g" --memory-swap="1g" --cpus="4" --ipc='shareable' ruby bash
but nothing helps.