[feature request] support passing -H / --host to docker sandbox

[JasonGross]

I'd like to be able to specify that some tasks / samples should run on a remote machine by passing -H to the docker commands. Right now, I am using os.environ["DOCKER_HOST"] = docker_host, but it seems like this might interfere with other tasks if I try to run them in parallel. Could support be added to bind -H on a per-task basis?

[jjallaire-aisi]

This is a good suggestion -- right our now SandboxSpec type is a simple tuple and I think it needs to become a richer data structure.