Fix build: docker needs -f Containerfile (default is Dockerfile)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.forgejo/workflows/build.yml

@@ -38,6 +38,7 @@ jobs:
       - name: Build image (uses host Docker via mounted socket)
         run: |
           docker build \
+            -f Containerfile \
             --build-arg BASE_IMAGE="${{ matrix.base_image }}" \
             --build-arg BASE_TAG=stable \
             -t "${IMAGE}:stable" \