Fix #77 groupmod added (non-unique -o)
Fix #77 groupmod added (non-unique -o)

```bash
groupmod -o ...
```
option ```-o``` allows groupmod for non-unique id's.

Result:
Before on HOST:
```bash
total 0
drwxr-xr-x  1  102  106  6 Aug 31 11:26 .
d---------+ 1 root root 70 Aug 31 11:37 ..
drwx------  1  102  106  8 Aug 19 15:49 9.5
```

After on HOST:
```bash
total 0
drwxr-xr-x  1 admin users  6 Aug 31 11:38 .
d---------+ 1 root  root  70 Aug 31 11:52 ..
drwx------  1 admin users  8 Aug 19 15:49 9.5
```

Docker log after fix #77
```log
Adapting uid and gid for postgres:postgres to 1024:100
Initializing datadir...
```
sameersbn/docker-postgresql,boky8/docker-postgresql,mental-docker/postgresql,markrowsoft/docker-postgres-bdr