Use Dockerfile with integrated build dependencies

This new dockerfile includes ghostscript, pip and apt so it should speed up the build significantly

.gitlab-ci.yml

@@ -3,12 +3,18 @@ variables:
   POSTGRES_USER: postgres
   POSTGRES_PASSWORD: ""
   CI_DEPS: psycopg2
+  # https://hub.docker.com/r/twiggers/python-thalia/
+  # https://github.com/thomwiggers/python-thalia
+  # Should get auto-updated with the official 'python' repository
+  # Installs:
+  #  - pip: tox, coverage
+  #  - apt: ghostscript
+  PY35_IMAGE: twiggers/python-thalia:3.5
 
 pep8:
-  image: python:3.5
+  image: $PY35_IMAGE
   before_script:
     - export PIP_CACHE_DIR="pip-cache"
-    - pip install tox
   script:
     - tox -e flake8
 
@@ -19,9 +25,6 @@ pep8:
     - export PIP_CACHE_DIR="pip-cache"
     - mkdir -p pip-cache
     - git log -1
-    - pip install tox coverage
-    - apt-get update
-    - apt-get install -y ghostscript  # required for Thabloid's tests
   script:
     - tox -e ${PYTHON_VERSION}-${DJANGO_VERSION}
     - cd website
@@ -29,13 +32,13 @@ pep8:
 
 python35-django11:
   <<: *djangotest
-  image: python:3.5
+  image: $PY35_IMAGE
   variables:
     PYTHON_VERSION: py35
     DJANGO_VERSION: django11
 
 docs:
-  image: python:3.5
+  image: $PY35_IMAGE
   before_script:
     # install django deps
     - pip install -r requirements.txt