Allow any uid/gid with USER_UID & USER_GID env var

If USER_UID and USER_GID are defined, ember will be run as a user with
these uid/gid, so the created files will not be owned by root.

Usage with `docker`:

    docker run -e USER_UID=1000 -e USER_GID=1000 <image>

Usage with `fig`, in fig.yml file:

    ember:
      image: <image>
      environment:
        - USER_UID=1000
        - USER_GID=1000