Create a docker file for building the documentation

.gitlab-ci.yml

@@ -39,3 +39,13 @@ build-fedora:
     - docker push openlp/fedora:latest
   only:
     - master@openlp/runners
+
+build-documentation:
+  stage: build
+  script:
+    - docker pull openlp/documentation || true
+    - docker build --cache-from openlp/documentation --tag openlp/documentation:$CI_COMMIT_SHA --tag openlp/documentation:latest -f Dockerfile.documentation .
+    - docker push openlp/documentation:$CI_COMMIT_SHA
+    - docker push openlp/documentation:latest
+  only:
+    - master@openlp/runners

Dockerfile.documentation

@@ -0,0 +1,3 @@
+FROM alpine:latest
+
+RUN apk --no-cache add py3-sphinx py3-sphinx_rtd_theme texlive-full