Build Docker images in GitHub workflow

[XanderVertegaal]

Currently, we're building our images on the server. With every new release, this causes the servers to slowly clutter up. The BO can manage by periodically running docker system prune, but this is not ideal.

We should look into building images on the server (as part of an automatic workflow, for instance). That will keep server loads low.

As an added benefit, it matches better how Kubernetes (our lord and saviour) will deal with Docker-based projects.