Automatic security scanning of the docker images

[gusmith]

Since #367 is closed (which should happen when #388 is merged), we have stopped using quay.io as repository for the entiy-service docker images, we are using dockerhub.

But dockerhub doesn't have the security scanning feature of quay.io.
We may want to find a solution to be able to get such scanning results without having to rely on quay.io.