server: Ensure that API base path is aligned across all occurences

[s-heppner]

Currently, we manually adapt the API base path (e.g., /api/3.1) across a couple of different occurrences:

• server/app/interfaces/discovery.py
• server/app/interfaces/registry.py
• server/app/interfaces/repository.py
• server/docker/discovery/Dockerfile
• server/docker/registry/Dockerfile
• server/docker/repository/Dockerfile
• (check for further occurrences)

We should

• (i) define a single point of truth, or
• (ii) ensure with a suiting test that all these occurrences are aligned