Add GitHub Actions CI workflow

[errordeveloper]

• Add Docker-oriented CI workflow with distinct Lint, Test & Build stages
• Use GitHub's polyglot linter
• Ensure tests are driven by docker-compose, which is at the centre current dev workflow
• Enable for images to push to a registry in the future

Closes #18

[errordeveloper]

@karenc this overlaps with what did in #17 and #25, let's discuss which way do you we want to go about this :)

When I started on this, I didn't realise you've opened #17 already...

[karenc]

Looking again at the run-tests workflow, I think the main advantage is just the simplicity of it. Even for people who are not familiar with github actions, you can quickly tell what it is trying to do.

One of the advantages of this PR is it tries to build the docker images. It can be especially useful as our set up become more complicated in the future.

[errordeveloper]

@karenc yeah, so if we stick to using same docker containers for development, testing and deployment, shouldn't have to deal with various dependencies issues in each of the environments separately. I do agree that run-tests workflow is quite simple as it stands.