ci: build prod image from .farhoodlabs/Dockerfile

Pulls the prod image up to the same toolset as the dev image (kubectl,
kubeseal, uv/uvx, forgejo CLIs, nano, vim) without diverging the upstream
root Dockerfile. Both build-dev.yml and build-prod.yml now share the same
fork-overlay Dockerfile; only the image tag and trigger branch differ.

.github/workflows/build-prod.yml

@@ -47,6 +47,7 @@ jobs:
         uses: docker/build-push-action@v6
         with:
           context: .
+          file: .farhoodlabs/Dockerfile
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}