Fix node and yarn version in danger image

This does two things:

1. Switch the ruby image to stretch
2. Install node and yarn

Cons:

• ruby:2.4.4-stretch is way bigger than ruby:2.4.4-alpine

Pros:

• We already have an install script for node on debian stretch in the repository.
• The size increase should hardly matter as our build images also come from ruby:2.4.4-stretch and therefore only the new node/danger layer needs to be downloaded:

REPOSITORY                                           TAG             SIZE                SHARED SIZE         UNIQUE SiZE
new-danger                                           latest          941.1MB             868.3MB             72.78MB
registry.gitlab.com/gitlab/gitlab-build-images       ruby-2.4.4-…    2.011GB             868.3MB             1.143GB
registry.gitlab.com/gitlab-org/gitlab-build-images   danger          113.8MB             0B                  113.8MB
ruby                                                 2.4.4-stretch   868.3MB             868.3MB             0B