Add danger image with ruby version 2.6.6

While working on https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/3769 which includes updating Danger and using gitlab-dangerfiles it turns out GitLab uses 2.7.4 while CustomersDot still uses 2.6.6.

Because the fix above includes running danger under the context of bundler, it's important that the ruby versions match. Currently the Dockerfile.danger file resides at ruby version 2.7.4 resulting in a ruby version mismatch in the danger pipeline.