docker: prepare compose for multiple test instances

docker/doc.md

@@ -2,3 +2,15 @@ The `php-fpm` process runs containerized.
 The php application always uses `/var/www` as it's work directory and home folder, and if `/var/www` is bind mounted it
 is necessary to adjust the path passed via FastCGI to `php-fpm` by changing the root directory to `/var/www`.
 This can achieved in nginx by setting the `fastcgi_param SCRIPT_FILENAME` to `/var/www/$fastcgi_script_name;`
+
+The default docker compose settings are intended for development and testing purposes.
+The folder structure expected by compose is as follows
+
+```
+<vichan-project>
+└── local-instances
+    └── 1
+        ├── mysql
+        └── www
+```
+The vichan container is by itself much less rigid.