fix(ci): use api/Dockerfile in build-and-push-api job

PR #111 fixed the build context to ./api but forgot to also update
the file path. The job was using ./Dockerfile (the frontend Dockerfile
which references nginx.conf and package-lock.json from the repo root),
causing the API image build to fail with a cache checksum error.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/ci.yml

@@ -335,7 +335,7 @@ jobs:
         uses: docker/build-push-action@v6
         with:
           context: ./api
-          file: ./Dockerfile
+          file: ./api/Dockerfile
           push: ${{ github.event_name == 'push' && github.ref == 'refs/heads/main' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}