cv requires /var/www/.cv directory to exist and be writeable

Need to do the following inside the container (as root):
```
mkdir /var/www/.cv && chown www-data:www-data /var/www/.cv
```

Otherwise things like cv upgrade:db fail with "Cannot create directory /var/www/.cv/upgrade" and cv ext:download fails with "Cannot create directory /var/www/.cv/state"