Previously a plugin which used these hooks would overwrite the top-level plugin
command's use of this hook, resulting in the dockerCli object not being fully
initialised.
Provide a function which plugins can use to chain to the required behaviour.
This required some fairly ugly arrangements to preserve state (which was
previously in-scope in `newPluginCOmmand`) to be used by the new function.
Signed-off-by: Ian Campbell <ijc@docker.com>
That is, the helper to be used from the plugin's `main`.
Also add a `helloworld` plugin example and build integration.
Signed-off-by: Ian Campbell <ijc@docker.com>
Ian Campbell