Merge #1908

1908: Optimize docs/Dockerfile r=mergify[bot] a=Erriez

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

## What type of PR?

Optimize/fix `docs/Dockerfile`

## What does this PR do?

### Related issue(s)
- Mention an issue like: #1851

## Prerequistes
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.

- [ ] In case of feature or enhancement: documentation updated accordingly
- [ ] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/workflow.html#changelog) entry file.

## Testing

The following tests has been executed locally:

```bash
export DOCKER_ORG=user
export DOCKER_PREFIX=
export MAILU_VERSION=master

cd tests
time docker-compose -f build.yml build --no-cache docs
	real	0m18.850s
	user	0m0.317s
	sys	0m0.124s

docker images
	REPOSITORY                   TAG               IMAGE ID       CREATED              SIZE
	user/docs                    master            3de6c8612cf3   19 seconds ago       38.1MB

docker run -it --rm --name mailu-docs -p 80:80 user/docs:master

Open web browser:
	Clear caches
	http://localhost:80/master
```

Co-authored-by: Erriez <Erriez@users.noreply.github.com>

docs/Dockerfile

@@ -1,19 +1,28 @@
-ARG DISTRO=python:3.8-alpine3.14
-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 \
- && pip3 install -r /requirements.txt
-
-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