The docker derivation builds the docker image, but it's only used by the nix system when the command:
nix docker-<component>-x86_64-linux is executed. The need is that the nix system gets only involved in the building of the docker image and let's the CI be in charge of publishing the docker image into the registry.
To do so, the build of the docker image needs to export the image so it's not in a tar.gz but as a docker tag.

So, this needs to be executed at the end of the build docker command.
docker load < result