Fix detect job checkout for workflow_dispatch builds

The detect job was checking out main instead of the dispatched ref,
so it couldn't find build files that only exist on feature branches.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.forgejo/workflows/build-container.yaml

@@ -30,6 +30,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ inputs.ref || github.sha }}
           fetch-depth: 2
 
       - name: Detect and classify changed containers