Dockerfile: add python3-sphinx

python3-sphinx is needed to build the htmldocs target.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

Dockerfile

@@ -66,6 +66,7 @@ RUN apt-get update && apt-get install -y \
 	python-pip \
 	python-pytest \
 	python-virtualenv \
+	python3-sphinx \
 	rpm2cpio \
 	sloccount \
 	sparse \