Update / Contribution: Impish based image

[Aterfax]

See my branch: https://github.com/Aterfax/docker-ubuntu-vnc-desktop/tree/impish

Refactored to work using the ubuntu rolling base image (Impish). Not extensively tested but appears to be working with passwords and SSL just fine.

Tested with Chrome, MobaXterm (VNC direct) and the following command:

sudo docker run --name docker-ubuntu-vnc-desktop-impish -e VNC_PASSWORD=mypassword -p 6081:443 -e SSL_PORT=443 -v ${PWD}/ssl:/etc/nginx/ssl -p 6080:80 -p 5900:5900 -v /dev/shm:/dev/shm docker-ubuntu-vnc-desktop-impish