"sudo docker push" fails with a seemingly bogus error message - docker

Question

Here is my terminal log (Ubuntu 22.04.1, Docker version 20.10.22, build 3a2c30b):
paul#desktop:~/work/arc/code$ docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
pauljurczak/arc latest 4f3f22791983 35 minutes ago 880MB
sub-1 latest 4f3f22791983 35 minutes ago 880MB
neo-1 latest 3dcf55bb7458 3 days ago 891MB
arc-tut latest 3a9aee91689b 4 days ago 230MB
paul#desktop:~/work/arc/code$ sudo docker push pauljurczak/arc
Using default tag: latest
The push refers to repository [docker.io/pauljurczak/arc]
An image does not exist locally with the tag: pauljurczak/arc
paul#desktop:~/work/arc/code$ sudo docker push pauljurczak/arc:latest
The push refers to repository [docker.io/pauljurczak/arc]
An image does not exist locally with the tag: pauljurczak/arc
pauljurczak/arc
paul#desktop:~/work/arc/code$ sudo docker push latest
Using default tag: latest
The push refers to repository [docker.io/library/latest]
An image does not exist locally with the tag: latest
I created pauljurczak/arc:latest image as shown there. I'm trying to push it to my pauljurczak/arc repository. The error messages don't make any sense to me. Why pauljurczak/arc in my push command is considered a tag? Why An image does not exist locally with the tag: latest, while there exists several images with that tag? What is happening here? I'm following push command description at https://docs.docker.com/docker-hub/repos/#pushing-a-docker-container-image-to-docker-hub.
When viewing my repository with Chrome, I see this info:
That's exactly what I was doing.
It seems that skipping sudo makes it work. Why is that?

Answer 1

It looks like you have both Docker Desktop and the standalone Docker Engine installed. This means you have two different Docker daemons running. The Docker Engine one is accessible via /var/run/docker.sock, given appropriate permissions; Docker Desktop runs a hidden Linux virtual machine (even on native Linux) and makes a similar socket file available in your home directory.
Docker Desktop uses the Docker CLI "context" feature to point docker at the socket in your home directory. That configuration is also stored in a file in your home directory.
This is where sudo makes a difference. When you run sudo docker ..., and it reads $HOME/.docker/contexts/, that home directory is now root's home directory, probably /root. That doesn't have any of the Docker Desktop related configuration in it, and so you use the default /var/run/docker.sock Docker Engine socket instead.
As you note, just consistently not using sudo will resolve this. (You could potentially need sudo to access the Docker Engine socket, which can all but trivially be used to root the host; the Docker Desktop VM is a little more isolated.) Uninstalling Docker Desktop and making sure to only use the standalone Docker Engine on native Linux also would resolve this.