[ruby/rails] Enable MN threads when running Puma (#10596)

Author: p8

frameworks/Ruby/rails/rails-mysql.dockerfile

@@ -6,6 +6,7 @@ EXPOSE 8080
 WORKDIR /rails
 
 # ENV RUBY_YJIT_ENABLE=1 YJIT is enabled in config/initializers/enable_yjit.rb
+ENV RUBY_MN_THREADS=1
 
 # Use Jemalloc
 RUN apt-get update && \