Custom user does not get set in islet config

When installing with custom user:
`make user-config USER=training PASS=blah`

Connect to islet account

```
Fatal: demo user not in docker group
Are you a new or existing user? [new/existing]: ^C
Trap received, exiting...
```

Fix is setting `USER=training` in `/etc/islet/islet.conf` but this should be automated.
