fix: Use fetch to ensure main branch exists before checkout

- Change from 'git checkout main' to 'git fetch origin main:main'
- Ensures main branch ref exists in detached HEAD state
- Fixes 'pathspec main did not match' error

🤖 Generated with Claude Code

.github/workflows/build_and_publish_docker.yml

@@ -122,8 +122,8 @@ jobs:
       if: github.ref_type == 'tag' || (github.event_name == 'repository_dispatch' && github.event.client_payload.pr_number != '')
       run: |
         # Checkout main branch to avoid detached HEAD when pushing
+        git fetch origin main:main
         git checkout main
-        git pull origin main
         # Extract the primary image tag for updating README
         if [[ "${{ github.ref_type }}" == "tag" ]]; then
           # For tag releases, use the version tag