feat: make the Drupal docroot configurable

Allow for setting a `docroot` directory for Drupal projects.

By default, I've set this to `web` as that's what the Drupal Composer
scaffold projects use, but other platforms such as Acquia Cloud hosting
use `docroot`.

This is how it can be configured in `build.yaml`:

```yaml
drupal:
  docroot: docroot
```

Fixes #2

templates/web/nginx/default.conf.twig

@@ -1,7 +1,7 @@
 server {
   server_name _;
 
-  root /app/docroot;
+  root /app/{{ drupal.docroot }};
 
   location / {
     try_files $uri /index.php?$query_string;