docker-images(grafana): manually create additional dir

Created by: bobheadxi

Grafana seems to be crash-loop-backoffing on dot-com trying to find/sg_grafana_additional_dashboards - looking at the diff of https://github.com/sourcegraph/sourcegraph/commit/8451bb7fa365b3d406fe51c6d447085978a7c322, I can't find anything that might cause this.

This change tries to create the dir manually in the image to avoid it being nonexisting, since on dot-com we don't seem to mount anything to /sg_grafana_additional_dashboards