Fixes not being able to add additional plugins at build time due to the
`grafana` volume being mounted over the existing `/var/lib/grafana`
directory. This is fixed by copying the dashboards into the container
from an existing bind mount instead of using the ``grafana`` volume.
This however leaves behind the volume which should be removed with
`docker volume rm grafana` or by setting `grafana_remove_old_volume` to
`True`.

Closes-Bug: #2039498
Change-Id: Ibcffa5d8922c470f655f447558d4a9c73b1ba361