Add support for different docker runtimes

[syphar]

After having looked at different docker runtimes for docs.rs I thought that this could be useful for the playground too.

Not as the final, perfect solution, but just as an incremental step where we get more isolation and security for little effort.

[syphar]

@shepmaster I got pointed to you by @Mark-Simulacrum , would you like to do this?

Should we keep it that simple or perhaps do things differently?

Where would the tests for this live?

Other usage examples, for rustwide:

• add support for gvisor as docker engine rustwide#138
• WIP: try sysbox rustwide#146