Optimize docs/Dockerfile

- Convert .rst to .html in temporary python:3.8-alpine3.14 build image
- Remove all unused packages
- Use nginx:1.21-alpine deployment image

docs/Dockerfile

@@ -1,20 +1,28 @@
-ARG DISTRO=alpine:3.8
-FROM $DISTRO
-
-COPY requirements.txt /requirements.txt
+# Convert .rst files to .html in temporary build container
+FROM python:3.8-alpine3.14 AS build
 
 ARG version=master
 ENV VERSION=$version
 
-RUN apk add --no-cache nginx curl python3 \
- && pip3 install -r /requirements.txt \
- && mkdir /run/nginx
-
-COPY ./nginx.conf /etc/nginx/conf.d/default.conf
+COPY requirements.txt /requirements.txt
 COPY . /docs
 
-RUN mkdir -p /build/$VERSION \
- && sphinx-build -W /docs /build/$VERSION
+RUN apk add --no-cache --virtual .build-deps \
+    gcc musl-dev \
+ && pip3 install -r /requirements.txt \
+ && mkdir -p /build/$VERSION \
+ && sphinx-build -W /docs /build/$VERSION \
+ && apk del .build-deps
+
+
+# Build nginx deployment image including generated html
+FROM nginx:1.21-alpine
+
+ARG version=master
+ENV VERSION=$version
+
+COPY ./nginx.conf /etc/nginx/conf.d/default.conf
+COPY --from=build /build/$VERSION /build/$VERSION
 
 EXPOSE 80/tcp