need curl for the latest pandoc install

Author: pahjbo

.github/docker/sphinx-build.Dockerfile

@@ -9,6 +9,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     ca-certificates \
     git \
     make \
+    curl \
     python3 \
     python3-pip \
     python3-venv \