Bug 1053940 - docker-worker: Encrypted environment variables

One open issue is the management of the gpg keys. Currently they are hard-coded.
`return reject` to avoid calling `resolve`... `Promise` will prevent the `resolve` call from having any effect, but let's just call it bad practice... It certainly invites a future bug...

Also not sure why you need to make a new Promise... You can just return one of those created... and use `Promise.resolve(undefined)` as return from in `else` flow...