Added rake task to create derivates after migration

lib/tasks/shrine_recreate_derivatives.rake

@@ -0,0 +1,12 @@
+namespace :debshots do
+  desc "Recreate derivatives (pre-rendered images in different sizes)"
+
+  task shrine_recreate_derivatives: :environment do
+
+    Screenshot.find_each do |screenshot|
+      puts "Screenshot… #{screenshot.id}"
+      screenshot.simage_derivatives!
+    end
+
+  end
+end