> is probably a copyright violation

No, it's not. The relevant language (which this post cites) from the Canonical IP policy deals with trademarks, not copyrights:

Any redistribution of modified versions of Ubuntu must be approved, certified or provided by Canonical if you are going to associate it with the Trademarks. Otherwise you must remove and replace the Trademarks and will need to recompile the source code to create your own binaries.

Canonical is trying to prevent third parties from passing off forks as genuine versions of Ubuntu. (Not that I agree with its approach, but that's what this language does.) Distributing a Docker image to your own web server doesn't "associate it with the Trademarks."

(edit: formatting)