Optimize the Dockerfile

The Dockerfile was created using another as a starting point. It is not clear that everything that is included in this Dockerfile is strictly necessary. It may also be able to use fewer layers and thus reduce the size of the image.

Start by reviewing the Dockerfile and identifying which included packages and layers are required for the Debian-based VNC, noVNC, and XFCE4 environment. Build the image before and after the cleanup, compare its size, and verify that the stated desktop and server environment still works.

Written by the indexing model from the issue text.