https://coderpad.io/ - It's a realtime tool for conducting interviews, for programmers. Docker lets me do a wide variety of tricky stuff involving the execution of untrusted code, like provide REPLs with very low latency.
Relying on containers for security is a recipe for none.
I didn't say solely :)
Very nice tool. I suggest including some popular libraries (modules) like Text::CSV for Perl.
In fact, every Perl program has already been written, it is just a 'use' away.