[Fuego] Fuego on Debian 10 / Jenkins 2.235.1 (and python 3.7 support)

Hello!

I spent a bit of time updating the Dockerfile to use Debian 10 (Buster) and
Jenkins 2.235.1 LTS.
I actually only wanted to get python3.7 instead of python3.5 ... but then
it was easier to go all the way.

I am still trying to verify I did not break anything.
Feel free to have a look / clone my branch hosted on gitlab and provide
feedback what I might be missing or alike.

https://gitlab.com/Lazlo/fuego/-/tree/feature/debian-10-jenkins-2.235.1-python3.7

I hope this is something that will help everyone.

Best,

Lazlo