Running PHP, Ruby or Python processes in sandbox mode

Running PHP, Ruby or Python processes in sandbox mode

I'm working on a small weekend project, which is basically an online IDE
that allows you to run PHP, Ruby or Python code from the browser. I have
everything setup and working, but the way i created the system, if a user
runs a badly-written script, or a script with heavy-calculation, the
system might slow down for everyone until i reach the timeout (15
seconds).
My system does not pass the fibonacci test. How can i run the process in
isolation, that would allow users to create:
while (true) { fibonacci() } // pseudo-code
Without crashing the server? I have considered the following courses of
action:
Running each process inside a Docker (https://www.docker.io) container,
but i'm not sure how docker deals with slow containers
Running each process inside a VM
Running each process in an instantly-created EC2 instance (which is not
really an option, since this is slow and expensive)