Use python3.7 as 3.6 image will stop being built nightly and will therefore go stale.

Dockerfile

 # The docker image built from this Dockerfile is intended only to run the test
 # suite and to build the documentation. Installation of this project should be
 # performed via pip.
-FROM registry.gitlab.developers.cam.ac.uk/uis/devops/infra/dockerimages/python:3.6-alpine
+FROM registry.gitlab.developers.cam.ac.uk/uis/devops/infra/dockerimages/python:3.7-alpine
 ADD ./ /usr/src/app/
 WORKDIR /usr/src/app
 RUN pip install tox && pip install -e .