Fix never policy message and ruby image version

What does this MR do and why?

Fixes https://gitlab.com/gitlab-org/gitlab/-/issues/455956

• Change test message for pull_policy: [never] to Pulling docker image ruby:latest so that it's not confused with the job trying to pull runner's image.
• Also switching from using ruby:2.6 to just use ruby:latest. Hopefully this is more stable, since sometimes the job seems to have problem pulling ruby:2.6

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Related to #455956