Support a root folder

As per https://pocketbase.io/docs/going-to-production/#using-docker will be nice to define a root folder `/pb` this will make more easy to define just one mount point:

```yml
services:
  pocketbase:
    image: ghcr.io/muchobien/pocketbase:latest
    container_name: pocketbase
    restart: unless-stopped
    volumes:
      - ./pb:/pb
```