Draft: Add Ruby 3.0 (and 2.7) triage-ops Docker images

What does this MR do and why?

This MR aims to produce Ruby 2.7 and 3.0 Docker images for triage-ops. In !480 (merged), custom support was added but instead of adding a new triage-ops-ruby-3.0, I followed the suggestion in README.md of:

In the interests of avoiding repetition, it's better to use the custom docker build if possible.

Where I presume 'custom docker build' is referring to Use a custom image with versioned features

This will assist gitlab-org/quality/triage-ops!1961 (merged).

Related: gitlab-org/quality/triage-ops!1961 (merged)

Checklist

• (If applicable) Add patches if required for upgrading Ruby version in https://gitlab.com/gitlab-org/gitlab-build-images/-/tree/master/patches/ruby.