Instale um servidor fictício:
sudo apt-get install xserver-xorg-video-dummy-lts-xenial
Adicione um arquivo /etc/X11/xorg.conf
com o seguinte conteúdo:
Section "Device"
Identifier "Configured Video Device"
Driver "dummy"
EndSection
Section "Monitor"
Identifier "Configured Monitor"
HorizSync 31.5-48.5
VertRefresh 50-70
EndSection
Section "Screen"
Identifier "Default Screen"
Monitor "Configured Monitor"
Device "Configured Video Device"
DefaultDepth 24
SubSection "Display"
Depth 24
Modes "1024x800"
EndSubSection
EndSection