I think the container host had some issues after updating local packages. After this build failure I started the last successful built container, which started, but I noticed issues with reaching out to external sources. Such as auto resolving topic titles based on URLs and the chat integration with Discord and Matrix failed. Rebooting the container host solved these issues. Then I retried a new container build, that goes passed the previous part that failed. So I think it’s fine now.
My container host does automatic restarts of services if a dependency got updated, maybe this caused issues on the host with incomplete restarts or dependencies that should cause a restart of a service. In any case, rebooting solved it.
Edit: The container was built this time successfully.