Configure a credential helper to remove this warning. See
https://docs.docker.com/engine/reference/commandline/login/#credentials-store

Login Succeeded
$ export IMAGE_TAG=${CI_COMMIT_TAG:-latest}
$ docker build -t $DOCKER_REGISTRY/$CI_PROJECT_NAMESPACE/$CI_PROJECT_NAME:$IMAGE_TAG .
time="2020-02-21T14:28:22Z" level=error msg="failed to dial gRPC: cannot connect to the Docker daemon. Is 'docker daemon' running on this host?: dial unix /var/run/docker.sock: connect: no such file or directory"
Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?
section_end:1582295385:build_script
section_start:1582295385:after_script
section_end:1582295385:after_script
section_start:1582295385:upload_artifacts_on_failure
section_end:1582295385:upload_artifacts_on_failure
ERROR: Job failed: command terminated with exit code 1