Rework static assets Dockerfile
Rework static assets Dockerfile

- Build the assets inside a container, but VOLUME mounted so
that the created files are saved on the host filesystem. This
will allow the nginx image to grab them at a later stage.

geotrellis/geotrellis-site,geotrellis/geotrellis-site,geotrellis/geotrellis-site,geotrellis/geotrellis-site