remove freeport, use docker to assign

[kfatehi]

there are too many issues with trying to use SSH and freeing a port, and making sure firewalls are open and all that -- i think it's better to do something along these lines:

docker run --rm -a stdin -a stdout -a stderr -i --workdir /data strider/strider-docker-slave bash

basically use the stream at that point to communicate with the container, piping this all back to the strider UI, and the --rm removes the container for us after the session is over too, taking care of #3 as well

[kfatehi]

dockerode makes this real easy, and demuxes the stream for us too -- i'll do this today.

[jaredly]

That's what I tried first, but you don't get the command's exitcode, which is important to strider

[jaredly]

also, I think we should keep the container around if the build fails, so you can inspect it.

[jaredly]

new impl doesn't use an ssh server