Allow executing without sudo

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 <[email protected]>

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