Just for reference: If you see the following error, it’s about time to follow the instructions posted above.
Err https://openhab.ci.cloudbees.com Packages
HttpError404
Get:53 https://openhab.ci.cloudbees.com Packages [369 B]
Err https://openhab.ci.cloudbees.com Packages
HttpError404
Fetched 9,105 kB in 21s (429 kB/s)
W: Failed to fetch https://openhab.ci.cloudbees.com/job/openHAB-Distribution/ws/distributions/openhab-offline/target/apt-repo/Packages HttpError404
W: Failed to fetch https://openhab.ci.cloudbees.com/job/openHAB-Distribution/ws/distributions/openhab-online/target/apt-repo/Packages HttpError404
E: Some index files failed to download. They have been ignored, or old ones used instead.