fix permissions after creating postgresql data direcory

i had an error when installing plugin related to permissions, this sets the permissions to the docker postgres user's id:gid.  i'm not sure if this is the best fix, as (at least on my system) the docker postgres uid matches up with 'syslog' and 'fuse' group.

```
+ chown dokku: /home/dokku/.postgresql
+ chown dokku: /home/dokku/.postgresql/admin_pw
+ docker run -v /home/dokku/.postgresql/data:/var/lib/postgresql/9.1/main jeffutter/postgresql su postgres -c '/usr/lib/postgresql/9.1/bin/postgres --single -D /var/lib/postgresql/9.1/main -c config_file=/etc/postgresql/9.1/main/postgresql.conf <<< "CREATE USER root WITH SUPERUSER PASSWORD '\''sUpErSeCrEt'\'';"'
2013-12-27 03:09:51 UTC FATAL:  could not open file "/var/lib/postgresql/9.1/main/PG_VERSION": Permission denied
```