Improve CI pipeline

Discuss how to improve the CI pipeline before merging it into master:

1. Testing (unit, integration, performance, etc.)
2. Building releases (slim jar, fat jar, data, fat jar + data?) - where to host these, as they are 100 MB+?
3. Static analysis (e.g. with PMD)
4. Code style checking (e.g. with checkstyle)
5. Building Docker images