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.