Extend GitHub Actions workflow to enable continuous deployment

[yogan]

Based on the GitHub Actions workflow introduced with #83, we shall extend this to fully automate a deployment based on Docker images.

This is what still needs to be done (for details, see ci-cd.md in the repo):

• build & publish nginx image
• build & publish frontend image
• create a docker-compose config that uses images from GitHub registry
• make sure pulling from GH reg. works (needs auth via token, see below);
  try to automate with a small script