Update docker file

Dockerfile

@@ -2,7 +2,7 @@ FROM python:3.9-alpine as sphinx-build-env
 RUN apk update
 RUN python -m pip install --upgrade pip
 RUN pip install -U sphinx
-RUN pip install -U sphinx-design
+RUN pip install -U sphinx_design
 RUN pip install sphinxcontrib-svg2pdfconverter
 RUN apk add librsvg
 RUN pip install sphinx_rtd_theme