Bug 1053940 - docker-worker: Encrypted environment variables

One open issue is the management of the gpg keys. Currently they are hard-coded.
Why this? you can chain promises here again... openpgp returns promises right?

Just do:
```js
return decryptMessage.then(function(plaintext) {
   var encryptedData = JSON.parse(plaintext);
   validateEncryptedData(encryptedData, taskId);
   return encryptedData.secretData;
}).catch(function(err) {
  this.runtime.log(err);
  throw err;
}.bind(this));  // <- you need to bind this here to use `this` in the promise handler...
```

No need to make promises... No need to try catch... I'm not sure what `this.runtime.log(err);` does. Either way, you certainly do need to make sure you have the current context (`this`).