docker.inside to rely on runC

docker.inside does a bind mount of the workspace, assuming the docker daemon is a local one. If we admit this assumption, then we should better rely on runC (docker 1.11 and later) that allows to exec inside a container with support for --env and --cwd, two options that `docker exec` doesn't support (and won't support, according to docker roadmap)

This would allow to avoid relying on `env` hack (https://github.com/jenkinsci/docker-workflow-plugin/blob/master/src/main/java/org/jenkinsci/plugins/docker/workflow/WithContainerStep.java#L225) and fix JENKINS-33510