Added a repo on DockerHub. Try changing the repo destination syntax.

2 jobs for master in 6 minutes and 48 seconds (queued for 1 second)
Name Stage Failure
failed
build_TreeMaker_cmssw_standalone_CMSSW_10_2_21_dockerhub Build
drwxrwxrwx    2 0        0           4.0K Apr  9 06:54 .
drwxrwxrwx 10 0 0 4.0K Apr 9 06:54 ..
-rw-rw-rw- 1 0 0 597 Apr 9 06:54 .gitlab-ci.yml
-rw-rw-rw- 1 0 0 2.0K Apr 9 06:54 Dockerfile
-rw-rw-rw- 1 0 0 2.6K Apr 9 06:54 Makefile
-rwxrwxrwx 1 0 0 229 Apr 9 06:54 entrypoint.sh
$ /kaniko/executor --context ${CI_PROJECT_DIR}/${CONTEXT_DIR} --dockerfile ${CI_PROJECT_DIR}/${CONTEXT_DIR}/Dockerfile --destination "${REGISTRY_IMAGE}/${IMAGE_NAME}:${CMSSW_VERSION}-${DATE}-${CI_COMMIT_SHORT_SHA}" --build-arg=BUILD_DATE="${DATE}" --build-arg=VERSION="${DATE}" --build-arg=VCS_URL="${CI_REPOSITORY_URL}" --build-arg=VCS_REF="${CI_COMMIT_SHORT_SHA}" --build-arg BASEIMAGE="${BASE_IMAGE}" --build-arg SCRAM_ARCH="${SCRAM_ARCH}" --build-arg CMSSW_VERSION="${CMSSW_VERSION}"
error checking push permissions -- make sure you entered the correct tag name, and that you are authenticated correctly, and try again: checking push permission for "index.docker.io/treemaker/cmssw:CMSSW_10_2_21-2020-04-09-034fa70f": UNAUTHORIZED: authentication required; [map[Class: Name:treemaker/cmssw Action:pull Type:repository] map[Action:push Type:repository Class: Name:treemaker/cmssw]]
ERROR: Job failed: exit code 1