Add experimental centos 7 docker file

The package list was tested over at 1, and I now adapted the other steps from the ubuntu craft image too. But I'm not sure how to test whether this docker image is usable by jenkins now? Can anyone tell me how I could test this?

I also noticed that there's now a devtoolset-8, which we may want to use instead of version 7 to get an even newer compiler. I'll experiment with that locally first and then update this file here.