Add new CodeOcean::File#read method

* With a new method, we can simplify our code to handle differences between file.content and file.native_file.read

spec/controllers/submissions_controller_spec.rb

@@ -131,7 +131,7 @@ describe SubmissionsController do
         expect_http_status(:ok)
 
         it 'renders the file content' do
-          expect(response.body).to eq(file.native_file.read)
+          expect(response.body).to eq(file.read)
         end
       end
 

spec/lib/runner/strategy/docker_container_pool_spec.rb

@@ -160,7 +160,7 @@ describe Runner::Strategy::DockerContainerPool do
       let(:files) { [build(:file, :image)] }
 
       it 'copies the file inside the workspace' do
-        expect(FileUtils).to receive(:cp).with(files.first.native_file.path, local_path.join(files.first.filepath))
+        expect(File).to receive(:write).with(local_path.join(files.first.filepath), files.first.read)
         container_pool.copy_files(files)
       end
     end

spec/models/code_ocean/file_spec.rb

@@ -56,4 +56,24 @@ describe CodeOcean::File do
       expect(file.errors[:weight]).to be_present
     end
   end
+
+  context 'with a native file' do
+    let(:file) { create(:file, :image) }
+
+    after { file.native_file.remove! }
+
+    context 'when the path has not been modified' do
+      it 'reads the native file' do
+        expect(file.read).to be_present
+      end
+    end
+
+    context 'when the path has been modified' do
+      before { file.update(native_file: '../../../../secrets.yml') }
+
+      it 'does not read the native file' do
+        expect(file.read).not_to be_present
+      end
+    end
+  end
 end