Add Dockerfile to build on Debian 12

[volatileman]

I tried to use openEMS from stable repo and it's missing SetStepExcite function in python interface also it's quite outdated (release from 2019 or so).
I tried to build openEMS on my Debian 12 host, but it has conflicts with libavutil-dev version since KDE requires v10, and through the dependencies tree openEMS requires v7.5.
So I built openEMS with Docker.
Please consider adding this Dockerfile to your repository

Dockerfile.txt

[thliebig]

I think this should be done in the openEMS-Project and it should use the already existing scripts?