Description
A plugin can't use the same name as one of the cli global flags (debug, log-level, context, ...), or worse can't use the same shortcut (ex: -c).

If I add a new flag to the hello-world plugin like this following:

flags.StringVarP(&compose, "compose", "c", "", "compose file")

and try to use it:

$ docker helloworld -c my-compose-file.yml
unable to resolve docker endpoint: context "my-compose-file.yml" does not exist

$ docker --help
...
  -c, --context string     Name of the context to use to connect to the daemon (overrides DOCKER_HOST env var and default context set with "docker context use")

This context flag is a root command flag, not a PersistentFlag and so shouldn't be used by the docker cli and forwarded directly to the plugin.