Request: specify custom resolution as a Docker environment variable

[NateTheGreat68]

I have a feature request that I've actually already implemented and done some initial testing on (for the armhf architecture, at least) in my own fork of the project: specifying the resolution via an environment variable when running the container.

My solution would be run like this (and is already mentioned in the README): docker run -p 8080:8080 -e RES=1600x900x24 solarkennedy/ipmi-kvm-docker

In the event that the resolution isn't specified, the current value of 1024x768x24 would be applied by default.