Add hack to fix missing /dev/fd in docker containers

This don't happen all the time but it seems building on the hub
throws warnings about this when installing RVM, so...

See: https://github.com/dotcloud/docker/issues/2356 and related
See: https://registry.hub.docker.com/u/jbbarth/ruby/build_id/6766/code/bd5vijfi5y85nfafkghvp8s/