add GitLab CI configuration

Add our standard GitLab CI configuration and tweak the project to be CI
friendly:

* A simple Dockerfile is added which builds an image which can run tox.
* The tox.ini is updated to save JUnit reports where our CI expects to
  find them and to add coverage XML reports.
* Remove the Circle CI badge from the README. We have GitLab CI badges
  set up for the entire UIS group in GitLab so we don't need any
  additional configuration.