CI: Push to gitlab.com registry instead of dockerhub (avoid rate limit errors) [#3516973]

Docker hub has significantly reduced rate limits for pulls as of 4/1, and also made unexpected changes to DA's dockerhub license.

Drupal CI is erroring due to limit - additional context + discussion in Drupal slack thread: https://drupal.slack.com/archives/CGKLP028K/p1743611728165519?thread_ts=...

Current images have been sync'd but need to update such that we build + push changes to this registry.

- Update docker login/auth to use credentials that can push to gitlab registry (https://gitlab.com/drupal-infrastructure/drupalci/drupalci-environments/...). I'll configure creds on that end, then need to setup CI vars in this project.
- Update registry we are pushing to to match.

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3516973-update-registry changes, plain diff MR !53
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