Allow executing without sudo (#113)

When release manager (gem owner) want to execute rake release,
it is assumed to be executed via sudo.

If owner belongs to docker group, it fails.

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Rakefile

@@ -22,7 +22,7 @@ namespace :docker do
   distros.each do |distro|
     task distro do
       puts "testing on #{distro}"
-      sh "sudo docker build . -f test/docker/Dockerfile.#{distro}"
+      sh "docker build . -f test/docker/Dockerfile.#{distro}"
     end
   end
 end