Dockerfile: add x11vnc
Dockerfile: add x11vnc

Fixes #2857

ehartsuyker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,heartsucker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,heartsucker/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop