Use image for bucket name with docker, if box is unset

This allows multiple machines using the image to share the cache
and avoids the following error:

    /Applications/Vagrant/embedded/lib/ruby/2.0.0/pathname.rb:389:in
    `initialize': no implicit conversion of nil into String (TypeError)

lib/vagrant-cachier/action/configure_bucket_root.rb

@@ -33,7 +33,14 @@ module VagrantPlugins
         def cache_root
           @cache_root ||= case @env[:machine].config.cache.scope.to_sym
             when :box
-              @env[:home_path].join('cache', box_name)
+              @box_name = box_name
+              # Box is optional with docker provider, so use image name if unset
+              if @box_name.nil? && @env[:machine].provider_name.to_sym == :docker
+                bucket_name = image_name.gsub(':', '-')
+              else
+                bucket_name = @box_name
+              end
+              @env[:home_path].join('cache', bucket_name)
             when :machine
               @env[:machine].data_dir.parent.join('cache')
             else
@@ -44,6 +51,10 @@ module VagrantPlugins
         def box_name
           @env[:machine].config.vm.box
         end
+
+        def image_name
+          @env[:machine].provider_config.image
+        end
       end
     end
   end