Dockerize build and test flow, and enable GitLab auto-DevOps

• Add Dockerfiles for generating environments in which to build/test the deliverables.
• Add Docker Compose configuration for running the build and test flows.
• And scripts for starting the build and test flows.
• Ensure build directory exists to avoid permission issues with build flow.
• Modify GitLab CI/CD configuration to run jobs for building the deliverables and running the tox tests (in containers using Docker-in-Docker).
• Enable full GitLab auto-DevOps jobs.
• Add coverage collection to the test job.
• Add example secrets file and modify the unit tests to use the secrets.
• Modify tox configuration to pass on secrets to the tests and to collect coverage data.
• Update the README with information about running the build and test flows via Docker Compose, about making a release, about setting up CI/CD and about secrets.
• Make PHP document generation depend on the required Docker image.
• Enable use of Docker inside the build container (required for building PHP documentation).
• Run build container as root user to avoid permissions issues with Docker. Force all writable files to be owned by the current user when the container exits.
• Fix use of Virtualenv when building Python documentation.
• Modify PHP documentation generation script to be able to run inside a container.
• Fix use of sed in the PHP documentation generation script.
• Override some SAST "spotbug" job options to use the build.
• Add extra line to generated Python methods file (lint fix).

Closes #11 (closed)