Last I checked, Docker containers are not hard to break out of unless you go to extra lengths (SELinux, AppArmor, etc--not entirely sure; I'm not an expert). Most people use Docker as a way to avoid their programs accessing the wrong version of a shared dependency or similar. I believe there may be other container runtimes with stronger guarantees or better defaults, some of which are even suitable for running untrusted code (or so they advertise).