lib/plugins/aws/invokeLocal/index.js/AwsInvokeLocal/pullDockerImage
class AwsInvokeLocal:  pullDockerImage() {
    const runtime = this.getRuntime();

    this.serverless.cli.log('Downloading base Docker image...');

    return new BbPromise((resolve, reject) => {
      const docker = spawn('docker', ['pull', `lambci/lambda:${runtime}`]);
      docker.on('exit', error => {
        return error ? reject(error) : resolve();
      });
    });
  }