Update Danger image dependency versions

Not sure how comfortable you are accepting MRs from external contributors for build dependencies, but we were looking for a Danger docker image and found that you offered one, but some of the dependencies are a little out of date.

• Ruby 2.7.4 - https://www.ruby-lang.org/en/news/2021/07/07/ruby-2-7-4-released/
• Node.js 14.17.2 (14.17.3 isn't available yet) - https://github.com/nodejs/node/blob/master/doc/changelogs/CHANGELOG_V14.md#14.17.2
• GitLab GEM 4.17.0 - https://github.com/NARKOZ/gitlab/releases/tag/v4.17.0
• Danger GEM 8.3.1 - https://github.com/danger/danger/blob/HEAD/CHANGELOG.md#831

There was also a small typo in the comment in the Dockerfile 😄