Fix redirect loop for subdirs multisite in nginx

The nginx.conf was missing some configuration for backends of individual WordCamps to work then using the year as a subdirectory, which is the default now.

Fixes WordPress#782

.docker/config/nginx.conf

@@ -36,6 +36,12 @@ server {
 		access_log off;
 	}
 
+	if (!-e $request_filename) {
+		rewrite /wp-admin$ $scheme://$host$request_uri/ permanent;
+		rewrite ^(/[^/]+)?(/wp-.*) $2 last;
+		rewrite ^(/[^/]+)?(/.*\.php) $2 last;
+	}
+
 	location / {
 		try_files $uri $uri/ /index.php?$args;
 	}