Use Dockerfiles instead of Docker image names for CI
We are currently using the python:x.y.z standard Docker images. We should refactor the Jenkinsfile to use Dockerfiles instead, so we can start testing in Jpp/ROOT environments too.
We are currently using the python:x.y.z standard Docker images. We should refactor the Jenkinsfile to use Dockerfiles instead, so we can start testing in Jpp/ROOT environments too.