I’m getting the same issue discussed here: https://help.octopusdeploy.com/discussions/problems/62012-problem-logging-into-internal-docker-registry running version 3.17.14. Looking through the release notes I can’t see anything to say this has been resolved.
Could you confirm that the bug still exists and if so raise a bug internally to resolve the issue.
Cheers.
Hi Matthew,
Thanks for getting in touch! I do not believe the customer in the referenced discussion attached the logs mentioned by Chris. Would you be able to do so? I think this may have been fixed but I do not know if it was entirely related to Octopus. The mentioned logs will help here. 
Could you please add the Docker registry to Octopus once again and when it fails, grab the logs from your registry and attach them to this support ticket?
Let me know if you have any questions or run into any problems.
Best regards,