Fix harbor registry documentation

Base64 line wrapping causes an invalid newline character. Doc page: https://docs.gitlab.com/ee/user/project/integrations/harbor.html

    - mkdir -p /kaniko/.docker
    - echo "{\"auths\":{\"${HARBOR_HOST}\":{\"auth\":\"$(echo -n ${HARBOR_USERNAME}:${HARBOR_PASSWORD} | base64)\"}}}" > /kaniko/.docker/config.json

---> note the base64 -w 0 command

    - mkdir -p /kaniko/.docker
    - echo "{\"auths\":{\"${HARBOR_HOST}\":{\"auth\":\"$(echo -n ${HARBOR_USERNAME}:${HARBOR_PASSWORD} | base64 -w 0)\"}}}" > /kaniko/.docker/config.json