Confirmed via a direct token request to Harbor's own token endpoint
(same username/password from harbor-robot-dockerconfig) that the
robot account genuinely has push access to homelab/demo-go-app —
Harbor returned a valid token with actions:[pull,push]. So the actual
`docker push` failure ("unauthorized... action: push") wasn't a
permissions problem at all.
Root cause: kubernetes.io/dockerconfigjson secrets are required to
store their data under the fixed key `.dockerconfigjson`. Mounting
the secret without remapping that key meant the file that actually
landed at /root/.docker was named `.dockerconfigjson`, not
`config.json` — the only filename docker's CLI reads for stored
credentials. Docker found nothing there and pushed unauthenticated,
which Harbor correctly rejected. Adds an items: remap so the mounted
file is named config.json.