CI: Optionally enable dependency proxy (Backport 17.8)

What does this MR do and why?

Backport !8258 (merged) to 17-8-stable to:

• Optionally pull all DockerHub images through the dependency proxy.
• Fix the invalid service image because the DOCKER_DIND_IMAGE is not defined on the stable branches.

Context https://gitlab.slack.com/archives/C1FCTU4BE/p1743574634652789 (internal; good for 90 days)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).