Support multiple docs versions

docs/Dockerfile

@@ -2,6 +2,9 @@ FROM python:3-alpine
 
 COPY requirements.txt /requirements.txt
 
+ARG version=master
+ENV VERSION=$version
+
 RUN pip install -r /requirements.txt \
  && apk add --no-cache nginx \
  && mkdir /run/nginx
@@ -9,6 +12,7 @@ RUN pip install -r /requirements.txt \
 COPY ./nginx.conf /etc/nginx/conf.d/default.conf
 COPY . /docs
 
-RUN sphinx-build /docs /build
+RUN mkdir -p /build/$VERSION \
+ && sphinx-build /docs /build/$VERSION
 
 CMD nginx -g "daemon off;"