Support for pulling Docker images

Customers are not able to pull private docker images form the GitLab container registry as authorisation is required. This is usually configured at a server level by adding credentials to the docker configuration. This cannot be done on GitHost. We should provide a dockercfg configuration option or other.

More information:

http://docs.gitlab.com/ce/ci/docker/using_docker_images.html#define-an-image-from-a-private-docker-registry

https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/blob/master/docs/configuration/advanced-configuration.md#using-a-private-docker-registry

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ZD: https://gitlab.zendesk.com/agent/tickets/29748

//cc @stanhu @dblessing