DB initialisation no longer works

Whilst running in the latest version of Docker on an Intel mac:

```
Attaching to containerized-guacamole-init-guac-db-1
containerized-guacamole-init-guac-db-1  | /bin/sh: 1: cannot create /init/initdb.sql: Permission denied
containerized-guacamole-init-guac-db-1 exited with code 2
```

My guess is something has changed in the Guacamole Docker image…