Add MariaDB to docker-compose example

1. Gives an example set up of a MySQL database in docker-compose
2. `php` now depends on `db`, and `nginx` depends on `php`. This
ensures the integrity of the stack.

docker-compose.yml.example

@@ -4,6 +4,8 @@ services:
 
   nginx:
     image: nginx
+    depends_on:
+      - php
     expose:
       - 80
     volumes:
@@ -12,7 +14,17 @@ services:
     
   php:
     build: .
+    depends_on:
+      - db
     expose:
       - 9000
     restart: always
 
+  db:
+    image: mariadb
+    restart: always
+    environment:
+      MYSQL_USER: crater
+      MYSQL_PASSWORD: crater
+      MYSQL_DATABASE: crater
+