Build extensions for Ruby 2.7 (#7027)

* Build extensions for Ruby 2.7

* Try installing bundler 2.x

* Try bumping rake-compiler-dock

* Use standard RCD images

* Avoid 'rake cross native' with rake-compiler-dock

* Use Ruby 2.5 for building Ruby <= 2.6

* Use rake-compiler 1.1.0

* Specify target

* Don't update Ruby test image for now
diff --git a/ruby/google-protobuf.gemspec b/ruby/google-protobuf.gemspec
index bf60b6a..cd9af71 100644
--- a/ruby/google-protobuf.gemspec
+++ b/ruby/google-protobuf.gemspec
@@ -17,13 +17,13 @@
   else
     s.files     += Dir.glob('ext/**/*')
     s.extensions= ["ext/google/protobuf_c/extconf.rb"]
-    s.add_development_dependency "rake-compiler-dock", "~> 0.6.0"
+    s.add_development_dependency "rake-compiler-dock", ">= 1.0.1", "< 2.0"
   end
   s.test_files  = ["tests/basic.rb",
                   "tests/stress.rb",
                   "tests/generated_code_test.rb"]
   s.required_ruby_version = '>= 2.3'
-  s.add_development_dependency "rake-compiler", "~> 0.9.5"
+  s.add_development_dependency "rake-compiler", "~> 1.1.0"
   s.add_development_dependency "test-unit", '~> 3.0', '>= 3.0.9'
   s.add_development_dependency "rubygems-tasks", "~> 0.2.4"
 end