Use a specific nikola Docker image to reduce build time

.gitlab-ci.yml

-image: python:3.5
-
-pages:
-  stage: 
-    build
-  script:
-    - apt-get update && apt-get install locales-all
-    - pip install "nikola[extras]"
-    - nikola build
-  artifacts:
-    paths:
-      - public
+image: mgarrido/nikola
+
+test:
+  script:
+  - nikola build
+  except:
+  - master
+
+pages:
+  script:
+    - nikola build
+  artifacts:
+    paths:
+    - public
+  only:
+  - master
\ No newline at end of file