Create Docker image based on OpenVisus image which includes web viewer

The current Docker deployments for OpenVisus include the web viewer, but that means upgrading the web viewer in OpenVisus requires rebuilding the OpenVisus image.

Perhaps a more appropriate solution is to create an image that includes the web viewer by basing it on the OpenVisus image (see sci-visus/ondemand for an example).

(related to https://github.com/sci-visus/OpenVisus/issues/52)