Update native_gem.rake according to example of rake-compiler-dock.

https://github.com/rake-compiler/rake-compiler-dock#add-to-your-rakefile

Author: andyundso

tasks/native_gem.rake

@@ -5,10 +5,10 @@ task 'gem:windows' => ['ports:cross'] do
   require 'rake_compiler_dock'
 
   # make sure to install our bundle
-  build = ['bundle']
+  sh "bundle package --all"   # Avoid repeated downloads of gems by using gem files from the host.
 
   # and finally build the native gem
-  build << 'rake cross native gem RUBY_CC_VERSION=2.7.0:2.6.0:2.5.0:2.4.0 CFLAGS="-Wall" MAKE="make -j`nproc`"'
-
-  RakeCompilerDock.sh build.join(' && ')
+  GEM_PLATFORM_HOSTS.keys.each do |plat|
+    RakeCompilerDock.sh "bundle --local && RUBY_CC_VERSION='2.7.0:2.6.0:2.5.0:2.4.0' CFLAGS='-Wall' MAKE='make -j`nproc`' rake native:#{plat} gem", platform: plat
+  end
 end