Base app image on openCARP

Hi! Since openCARP uses ubuntu18.04 as base, we could use the openCARP docker image as base for the app/flask container.

Then the docker call inside the app container is not needed anymore, it has not to run in privileged mode, and we don't need the /usr/local/bin/docker lines in docker-compose.yml. I think calling docker from flask has also security implications ...

I tried it and prepared a pull request (!3 (merged)), but somebody else should check carefully if it actually works.

Adding @axel.loewe @yung-lin.huang @D_Serra @rafael.sebastian .