Use saas-linux-medium-amd64 runners for all Docker single builds

What does this MR do and why?

Builds upon !4399 (34834562) and adds the same tags to the Docker build jobs as well.

Not only the test jobs fail, for example, here's one that pushes to registry https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/5577719725

Merge request acceptance checklist

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• I have evaluated the MR acceptance checklist for this merge request.