BUGFIX: Allow local docs rendering

* Update .gitlab-ci.yml to allow running
  gitlab-runner exec docker render:documentation
  locally

.gitlab-ci.yml

@@ -1,5 +1,3 @@
-image: 'bash:latest' 
-
 before_script:
   - apk add --no-cache make wget git
   - make install-composer
@@ -12,6 +10,7 @@ stages:
   - render
 
 sync:github:
+  image: 'bash:latest' 
   stage: sync
   before_script:
     - apk add --no-cache openssh-client git
@@ -68,7 +67,7 @@ test:latest:
     - ./vendor/bin/phpunit
 
 render:documentation:
-  image: danielsiepmann/sphinx:latest
+  image: 'danielsiepmann/sphinx:latest'
   stage: render
   before_script:
     - cd Documentation