Another entry into the hall of fame for worst error (unless you understand what is happening under the covers!).

This is when attempting to login to a container registry to push a docker image. The same code worked earlier, the only difference is that I have since installed docker-compose. Sure enough, found this thread:

The basic gist is that the ubuntu package for docker-compose is junk. It has a dependency on some golang provider or other which terribly has a dependency on X11 which is not present on a headless server. Yuck.

Although I had already uninstalled docker-compose with apt and manually installed the latest version earlier, I ALSO had to make sure that I ran apt autoremove to remove some of the packages that were installed by docker-compose but not automatically removed. Once I did that, it worked fine without the horrible GPG type workaround!