Hello Ivan,
in general it is possible to get the repository services from the container in your plugin base class.
But you try to get a repository which is provided by your plugin. The container is only build with installed and activated plugin.
So in the request where you try to install/activate your plugin, the service is of course not in the container. It would be first available in the very next request.
Best regards from Schöppingen
Michael Telgmann