Add focal-based nvidia docker image

This MR adds focal-based nvidia support images.

Compared to 16.04 nvidia image Dockerfile, I removed the following line:

COPY --from=nvidia /etc/ld.so.conf.d/glvnd.conf /etc/ld.so.conf.d/glvnd.conf

The file /etc/ld.so.conf.d/glvnd.conf is not existent in nvidia/opengl:1.0-glvnd-runtime-ubuntu20.04 anymore.

Contributes to clickable#277 (closed).