Add rake-compiler-dock as a dep.

ruby/Gemfile.lock

@@ -10,6 +10,7 @@ GEM
     rake (10.4.2)
     rake-compiler (0.9.5)
       rake
+    rake-compiler-dock (0.5.1)
     rubygems-tasks (0.2.4)
     test-unit (3.0.9)
       power_assert
@@ -21,6 +22,7 @@ PLATFORMS
 DEPENDENCIES
   google-protobuf!
   rake-compiler
+  rake-compiler-dock
   rubygems-tasks
   test-unit
 

ruby/google-protobuf.gemspec

@@ -9,11 +9,12 @@ Gem::Specification.new do |s|
   s.email       = "protobuf@googlegroups.com"
   s.require_paths = ["lib"]
   s.files       = `git ls-files -z`.split("\x0").find_all{|f| f =~ /lib\/.+\.rb/}
-  unless RUBY_PLATFORM == "java"
+  if RUBY_PLATFORM == "java"
+    s.files     += ["lib/google/protobuf_java.jar"]
+  else
     s.files     += `git ls-files "*.c" "*.h" extconf.rb Makefile`.split
     s.extensions= ["ext/google/protobuf_c/extconf.rb"]
-  else
+    s.add_development_dependency "rake-compiler-dock"
-    s.files     += ["lib/google/protobuf_java.jar"]
   end
   s.test_files  = ["tests/basic.rb",
                   "tests/stress.rb",