The dummy package without the patch like is used in the package for xf86-video-dummy-with-vt will cause problems with the console and X session interaction. It might be a good idea to compile it manually and apply the patch.
For setup, Stormux has a file in /etc/X11/xorg.conf.d. I don't think markdown is supported here, so the contents will probably not come out right, but here it is:
10-headless.conf
Section "Monitor"
Identifier "dummy_monitor"
HorizSync 28.0-80.0
VertRefresh 48.0-75.0
Modeline "1920x1080" 172.80 1920 2040 2248 2576 1080 1081 1084 1118
EndSection
Section "Device"
Identifier "dummy_card"
VideoRam 256000
Driver "dummy"
EndSection
Section "Screen"
Identifier "dummy_screen"
Device "dummy_card"
Monitor "dummy_monitor"
SubSection "Display"
EndSubSection
EndSection