Add docker stack for building documentation locally.

.gitattributes

@@ -8,3 +8,5 @@ phpunit.xml.dist export-ignore
 infection.json.dist export-ignore
 grumphp.yml.dist export-ignore
 phpspec.yml.dist export-ignore
+/docker export-ignore
+docker-composer.yaml export-ignore

docker-compose.yaml

@@ -0,0 +1,10 @@
+version: "3"
+
+services:
+    doc:
+        container_name: sphinx_documentation
+        build: docker/doc
+        volumes:
+            - ./docs:/etc/Sphinx/source
+        ports:
+            - 8100:80

docker/doc/Dockerfile

@@ -0,0 +1,11 @@
+FROM alpine:latest
+WORKDIR /etc/
+RUN mkdir -p /etc/Sphinx/build
+
+RUN apk add --no-cache python3 py3-pip make git
+
+RUN pip3 install git+https://github.com/sphinx-doc/sphinx
+RUN pip3 install sphinx-autobuild
+RUN pip3 install sphinx-rtd-theme
+
+CMD sphinx-autobuild -b html --host 0.0.0.0 --port 80 /etc/Sphinx/source /etc/Sphinx/build