Add support for configuring xvfb dpi and scale

[CaffeinatedCM]

🚀 Feature Proposal

Add SCREEN_DPI and SCREEN_SCALE settings to provide greater control of the xvfb display

Motivation

We recently discovered a bug in our product and would like to be able to automate testing of different screen scales and dpi settings with the docker containers.

Example

Testing and validating how websites behave when used on a 4k display where the screen is being scaled

[ddavison]

could you elaborate on what these settings in particular would change as far as the Xvfb run is concerned?

https://github.com/SeleniumHQ/docker-selenium/blob/master/NodeBase/start-xvfb.sh

perhaps we could just open up specifying options? something like XVFB_OPTIONS=-screen 1024x768x32 -pixdepths 32 then just pass that into the command

[CaffeinatedCM]

It looks like opening up specifying those options will help for setting the DPI, since xserver accepts that option.

I did some more research, and setting screen scale appears to have to be done using xrandr after the server is started, so perhaps we could add a call to xrandr if an environment variable XRANDR_OPTIONS is supplied?

[diemol]

@CaffeinatedCM it is hard to say. Would you like to give it a try and send us a PR which includes documentation?

[CaffeinatedCM]

@diemol sure, I can do that 😄

[diemol]

Closing this one and will follow up in the PR, thanks @CaffeinatedCM