Lock rake-compiler-dock for compatibility with Dockerfiles

Author: ianks

gem/rb_sys.gemspec

@@ -28,7 +28,7 @@ Gem::Specification.new do |spec|
   end
 
   # Needed for `RakeCompilerDock.cross_rubies`
-  spec.add_dependency("rake-compiler-dock", ">= 1.9.1")
+  spec.add_dependency("rake-compiler-dock", "1.9.1")
 
   spec.metadata["rubygems_mfa_required"] = "true"
 end