Add docker.container.image on docker/cpu stats (Metricbeat)

[kierenj]

When filtering for metricset.module:docker AND metricset.name:cpu, I get a docker.container.name but no docker.container.image populated. Could we add it please?

As per @exekias ' comment here: https://discuss.elastic.co/t/no-docker-container-image-on-docker-cpu-stats/147169

Thank you!