We present a novel emulation system for creating high-fidelity digital twins of IT infrastructures. The digital twins replicate key functionality of the corresponding infrastructures and allow to play out security scenarios in a safe environment. We show that this capability can be used to automate the process of finding effective security policies for a target infrastructure. In our approach, a digital twin of the target infrastructure is used to run security scenarios and collect data. The collected data is then used to instantiate simulations of Markov decision processes and learn effective policies through reinforcement learning, whose performances are validated in the digital twin. This closed-loop learning process executes iteratively and provides continuously evolving and improving security policies. We apply our approach to an intrusion response scenario. Our results show that the digital twin provides the necessary evaluative feedback to learn near-optimal intrusion response policies.