This is to make the custom dockerfiles compatible with our instruction
to use persitent volumes for the /var/lib/grafana dir. Without this
change the contents of that folder will be overshadowed by the persitent
volume and none of the pre-installed plugins will be available.
Adds support for using custom dockerfiles to pre-install image
renderer plugins.
Updates docs for docker and image rendering.
Fixes#20241
Co-Authored-By: Leonard Gram <leo@xlson.com>
