Repl.it CEO here. As @tony-allan mentioned we're indeed container based. Containers, however, on their own aren't safe for isolation. They're NOT sandboxes. You need to do a lot of things around containers to secure them. Most commonly is housing them in a VM.
At Repl.it we have multiple layers of security with the most important being that the entire execution environment has access to zero user data. Even if someone escaped the container jail, and the VM jail, and compromised the server they really couldn't do much beyond that.
We also use and built a bunch of intrusion detection software and manually look through potential threats.
Finally, if you want a container-based sandbox take a look at GVisor by Google. It's still an early project but has a lot of potential.