space manifests will error when setting both docker and buildpacks

[Finishes #163660668]

app/messages/app_manifest_message.rb

@@ -56,6 +56,7 @@ def self.underscore_keys(hash)
     validate :validate_app_update_message!
     validate :validate_buildpack_and_buildpacks_combination!
     validate :validate_docker_enabled!
+    validate :validate_docker_buildpacks_combination!
     validate :validate_service_bindings_message!, if: proc { |record| record.requested?(:services) }
     validate :validate_env_update_message!, if: proc { |record| record.requested?(:env) }
     validate :validate_manifest_singular_buildpack_message!, if: proc { |record| record.requested?(:buildpack) }
@@ -369,6 +370,12 @@ def validate_docker_enabled!
       errors.add(:base, e.message)
     end
 
+    def validate_docker_buildpacks_combination!
+      if requested?(:docker) && (requested?(:buildpack) || requested?(:buildpacks))
+        errors.add(:base, 'Cannot specify both buildpack(s) and docker keys')
+      end
+    end
+
     def add_process_error!(error_message, type)
       errors.add(:base, %(Process "#{type}": #{error_message}))
     end

spec/unit/messages/app_manifest_message_spec.rb

@@ -577,6 +577,43 @@ module VCAP::CloudController
         end
       end
 
+      describe 'combination errors' do
+        context 'when docker and buildpack is provided' do
+          before do
+            FeatureFlag.make(name: 'diego_docker', enabled: true, error_message: nil)
+          end
+
+          let(:buildpack) { Buildpack.make }
+          let(:params) { { buildpack: buildpack.name, docker: { image: 'my/image' } } }
+
+          it 'is not valid' do
+            message = AppManifestMessage.create_from_yml(params)
+
+            expect(message).not_to be_valid
+            expect(message.errors.count).to eq(1)
+            expect(message.errors.full_messages).to include('Cannot specify both buildpack(s) and docker keys')
+          end
+        end
+
+        context 'when docker and buildpacks is provided' do
+          before do
+            FeatureFlag.make(name: 'diego_docker', enabled: true, error_message: nil)
+          end
+
+          let(:buildpack) { Buildpack.make }
+          let(:buildpack2) { Buildpack.make }
+          let(:params) { { buildpacks: [buildpack.name, buildpack2.name], docker: { image: 'my/image' } } }
+
+          it 'is not valid' do
+            message = AppManifestMessage.create_from_yml(params)
+
+            expect(message).not_to be_valid
+            expect(message.errors.count).to eq(1)
+            expect(message.errors.full_messages).to include('Cannot specify both buildpack(s) and docker keys')
+          end
+        end
+      end
+
       context 'when there are multiple errors' do
         let(:params) do
           {