Ability to use private docker images

Currently if the image is private on docker hub and requires login, dunner fails with error message as: 

```
docker: checking existence of the image 'ngnix:1.17.3'
Pulling image: 'ngnix:1.17.3'... -DEBU[2019-08-26 18:09:20] Error response from daemon: pull access denied for ngnix, repository does not exist or may require 'docker login'
```

