Update docker-compose variables

docker-compose.yml

@@ -3,7 +3,7 @@ services:
   db:
     image: mariadb
     restart: always
-    user: 1000:1000
+    user: ${USER_GROUP}
     volumes:
       - ./db:/var/lib/mysql
   drupal:

env.sample

@@ -1,3 +1,5 @@
+USER_GROUP=1000:1000
+
 MARIADB_RANDOM_ROOT_PASSWORD=true
 MARIADB_DATABASE=artxibo
 MARIADB_USER=artxibo