Add newline to POSTGRES_PASSWORD file for initdb

#1024 converted all
`echo` calls to `printf`, but this change causes the password file
used by `initdb` to be blank rather than contain a single newline.
As a result, `initdb` will fail to start with an empty value with
the error:

```
initdb: error: password file "/dev/fd/63" is empty
```

`POSTGRES_PASSWORD` can be blank if `POSTGRES_HOST_AUTH_METHOD=trust`
is used. This change adds a newline to restore the original behavior.

Closes #1025

Author: stanhu