As the `--label` option is used before in `docker node update --label-add registry=true node1`, the Docker registry should be restricted to only run on `node1` - and nowhere else. So the `docker service create` command has to use the option `--constraint 'node.labels.registry==true` instead of `--label registry=true`- because it is a contraint, where to run the Registry - we don´t just want to set a label again.
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