Use Docker with Github Actions for building

[Techno-coder]

This PR depends on #79 and should be rebased after the dependency is merged.

This PR includes:

• Immediate replacement of Travis CI with Github Actions
• Building with Docker which reduces build times and fixes Travis CI is slow #72

Prerequisites:

• A Docker Hub account is required for storing cached image artefacts. Account details are specified in the repository secrets as DOCKER_USERNAME and DOCKER_PASSWORD

As an additional benefit, the built Docker image can be retrieved from the registry and downloaded locally for testing or deployment. The image follows the form of <username>/nztrain:latest.