feat: add GitLab runner provider

[jonschumaker]

Summary

Bring the tested GitLab runner-provider implementation into this fork's main branch.

This adds CI_PROVIDER=github|gitlab, preserves GitHub as the default, and runs GitLab jobs through official GitLab Runner managers with persistent system IDs and private per-slot Docker executors.

Impact

• Existing GitHub behavior and credentials remain compatible.
• GitLab.com and self-managed GitLab can use modern glrt- runner tokens.
• GitLab job Docker access stays on a private Unix socket by default; no Docker TCP API or Unraid host socket is exposed.
• Dynamix settings, fleet status, packaging, deployment, documentation, and provider-specific credentials are updated.

Validation

• bash tests/run-linux-checks.sh passed.
• Generated GitLab Runner configuration parsed successfully with the official runner image.
• Real privileged DinD validation confirmed Unix-socket operation with no listeners on ports 2375/2376.
• An Unraid smoke test completed 5,301 backend tests and 179 frontend test files with autoscaling and configured CPU/memory limits.
• Post-job cleanup left no executor containers, restarts, or OOM events.

The same changes are proposed upstream in unraid#44.