add adminer to docker-compose

docker-compose.yml

@@ -29,6 +29,12 @@ services:
             MYSQL_DATABASE: 'mapguesser'
             MYSQL_USER: 'mapguesser'
             MYSQL_PASSWORD: 'mapguesser'
+    adminer:
+        image: adminer:4.8.1-standalone
+        ports:
+        - 9090:8080
+        environment:
+            - ADMINER_DEFAULT_SERVER=mariadb
     mail:
         image: marcopas/docker-mailslurper:latest
         ports: