I had exactly the same issue and could not find any solutions on the forum or the internet. So I did some digging myself and found the cause of the issue. The docker update check script gets the remote digest of the latest tag from the docker repository via a header called 'Docker-Content-Digest'. The script checks for this header with a case-sensitive regex pattern. Manually querying the docker hub registry gives me a header called 'docker-content-digest' (mind the casing). The docker hub regis