postgresql: Declare /var/lib/postgresql a VOLUME

Avoid the hassle of maintaining a host-mounted volume by letting
Docker handle the volume maintenance ;).  We need to declare the
VOLUME *after* filling it with content (with 'emerge --config'),
otherwise ownership and permissions on the empty volume are lost
[1,2,3], and future RUN commands die due to:

  initdb: could not access directory "/var/lib/postgresql/9.3/data": Permission denied

[1]: https://github.com/dotcloud/docker/issues/2360
[2]: https://github.com/dotcloud/docker/issues/2969
[3]: https://github.com/dotcloud/docker/issues/2975
[4]: https://github.com/dotcloud/docker/pull/3008