chore: Update CI workflow to specify self-hosted runner configuration

### Summary of Changes
- Modified the CI workflow for Docker builds to explicitly define the runner configuration as `runs-on: self-hosted` for the setup job, enhancing clarity in the workflow setup.

### Expected Results
- Improved readability and maintainability of the CI configuration, ensuring that the intended execution environment is clearly specified.

.gitea/workflows/docker-build.yml

@@ -15,7 +15,8 @@ env:
 
 jobs:
   # Shared setup job for common steps
-  setup: self-hosted
+  setup:
+    runs-on: self-hosted
     outputs:
       version: ${{ steps.meta.outputs.version }}
       labels: ${{ steps.meta.outputs.labels }}