Duplicate the context hash when duplicating an Attacher

lib/shrine/attacher.rb

@@ -341,6 +341,13 @@ def shrine_class
 
       private
 
+      # The copy constructor that's called on #dup and #clone
+      # We need to duplicate the context to prevent it from being shared
+      def initialize_copy(other)
+        super
+        @context = @context.dup
+      end
+
       # Converts a String or Hash value into an UploadedFile object and ensures
       # it's uploaded to temporary storage.
       #

test/attacher_test.rb

@@ -6,6 +6,13 @@
     @shrine   = @attacher.shrine_class
   end
 
+  describe '#initialize_copy' do
+    it 'duplicates the context when duplicating the attacher' do
+      attacher_copy = @attacher.dup
+      refute_equal @attacher.context.object_id, attacher_copy.context.object_id
+    end
+  end
+
   describe ".from_data" do
     it "instantiates an attacher from file data" do
       file     = @attacher.upload(fakeio)

test/plugin/model_test.rb

@@ -136,6 +136,7 @@
         assert_equal model_copy, model_copy.file_attacher.record
         assert_equal :file, model_copy.file_attacher.name
         assert_equal Hash[record: model_copy, name: :file], model_copy.file_attacher.context
+        assert_equal Hash[record: model, name: :file], model.file_attacher.context
         assert model_copy.file_attacher.changed? # retains any state
       end