A behaviour for a plugin system allowing functionality to be extended when building the docker image.

By implementing the optional callbacks the docker file can be changed at various points.

All callbacks are optional.

Creating a plugin

1. Create module e.g. MyProject.MyPlugin
2. Add use DockerBuild.Plugins
3. Implement the required callbacks to modify the docker file or .dockerignore
4. To fetch a plugin config value with the plugin callback use plugin_config(context, key) where context is either the config or df parameter passed to the callback.