Add retries to working_docker_image?

* Also synchronize images during save

spec/models/execution_environment_spec.rb

@@ -8,7 +8,7 @@ describe ExecutionEnvironment do
   it 'validates that the Docker image works' do
     allow(execution_environment).to receive(:validate_docker_image?).and_return(true)
     allow(execution_environment).to receive(:working_docker_image?).and_return(true)
-    execution_environment.update(docker_image: FactoryBot.attributes_for(:ruby)[:docker_image])
+    execution_environment.update(FactoryBot.build(:ruby).attributes)
     expect(execution_environment).to have_received(:working_docker_image?)
   end
 
@@ -146,10 +146,12 @@ describe ExecutionEnvironment do
   end
 
   describe '#working_docker_image?' do
+    let(:execution_environment) { FactoryBot.create(:ruby) }
     let(:working_docker_image?) { execution_environment.send(:working_docker_image?) }
     let(:runner) { instance_double 'runner' }
 
     before do
+      allow(execution_environment).to receive(:sync_runner_environment).and_return(true)
       allow(Runner).to receive(:for).with(execution_environment.author, execution_environment).and_return runner
     end
 
@@ -176,7 +178,7 @@ describe ExecutionEnvironment do
     context 'when the Docker client produces an error' do
       it 'adds an error' do
         allow(runner).to receive(:execute_command).and_raise(Runner::Error)
-        working_docker_image?
+        expect { working_docker_image? }.to raise_error(ActiveRecord::RecordInvalid)
         expect(execution_environment.errors[:docker_image]).to be_present
       end
     end