Make sure your logging into the right area… I had this using GitLab’s CI, each docker login passed but the push failed.
I changed the process to do this, and it solved it for me…
docker login -u "myusername" -p "mypassword" docker.io
docker push myusername/myimage:0.0.1
I also added myusername as a contributer to by docker hub repo. I’m not sure whether or not this final step is needed as I was trying everything at the time to get it to work.