Upgrade sphinx and base image

Author: simonswine

docs/Dockerfile

@@ -1,10 +1,12 @@
 # Copyright Jetstack Ltd. See LICENSE for details.
-FROM python:3
+FROM python:3.6-stretch
 
 WORKDIR /site
 
+# ensure python and dependencies are installed
 RUN apt-get update && apt-get install -y python-enchant wbritish
 
+# install sphinx
 COPY requirements.txt .
 RUN pip install -r requirements.txt
 RUN mkdir -p venv/bin && ln -s $(which python) venv/bin/python && touch .venv

docs/requirements.txt

@@ -1,5 +1,5 @@
-CommonMark==0.5.4
-Sphinx==1.6.3
-sphinx-rtd-theme==0.2.4
-sphinxcontrib-spelling==2.3.0
+recommonmark==0.4.0
+sphinx==1.7.5
+sphinx-rtd-theme==0.4.0
+sphinxcontrib-spelling==4.1.0
 sphinxcontrib-versioning==2.2.1