Allow port forwarding to docker containers from docker executor



Currently you cannot build and run a docker container from within the docker executor type itself, as the primary container runs on a different remote docker network to the ones launched from the container (after setup_remote_docker is invoked), I know this is difficult to get right but it would be incredibly handy if this could made to work natively without reverting to machine executor and doing all the docker steps manually, as part of the attraction for using CircleCI 2.0 is it’s inbuilt first class docker support, such as easily setting up containers for jobs with DB etc. in them, while using the machine executor means doing everything ourselves.

Support resource_class for machine executor

@Circleci - are you going to do support this?

closed #3