Adding a `docker exec` option

[jeff-kilbride]

@mhart First, thanks for all the contributions! I've used / gotten inspiration from several of your node-based containers from docker hub.

When I'm testing, I prefer to start the container once and keep it running, rather than executing docker run each time. With that in mind, I made some simple changes to docker-lambda to allow the use of docker exec on a running container. Here's the relevant piece:

  options = options || {};
  var dockerImage = options.dockerImage || 'lambci/lambda:nodejs4.3';
  var handler = options.handler || 'index.handler';
  var event = options.event || {};
  var taskDir = options.taskDir == null ? process.cwd() : options.taskDir;
  var cleanUp = options.cleanUp == null ? true : options.cleanUp;
  var addEnvVars = options.addEnvVars || false;
  var dockerCmd = options.dockerCmd || 'run';
  var dockerArgs = options.dockerArgs || [];
  var spawnOptions = options.spawnOptions || { encoding: 'utf8' };
  var returnSpawnResult = options.returnSpawnResult || false;

  var args = [dockerCmd];
  if (dockerCmd === 'run') {
    args = args
      .concat(taskDir ? ['-v', taskDir + ':/var/task'] : [])
      .concat(cleanUp ? ['--rm'] : [])
      .concat(addEnvVars ? ENV_ARGS : [])
      .concat(dockerArgs)
      .concat([dockerImage, handler, JSON.stringify(event)]);
  } else {
    args = args
      .concat(addEnvVars ? ENV_ARGS : [])
      .concat(dockerArgs)
      .concat([
        'node',
        '--expose-gc',
        '--max-semi-space-size=150',
        '--max-old-space-size=2707',
        '/var/runtime/node_modules/awslambda/index.js',
      ])
      .concat([handler, JSON.stringify(event)]);
  }

I added a dockerCmd option that accepts either run or exec, and defaults to run. When using exec, the dockerArgs option takes the name of the running container. I then just execute the same command that you have in your entrypoint config.

It's not a huge difference, but using exec cuts about a second off each run on a basic hello world type test. If you're interested, I can create a PR for this -- or you can just copy and paste the above. Everything else is identical.

Ideally, to really speed up testing, I'd like to keep the node process running the same way lambda does. Do you have any insight on how I might go about that?