Use linuxserver/mariadb for the database

The official MySQL Docker image doesn't provide any ARM variants.
Use the linuxserver/mariadb which supports all of them.

readme-vars.yml

@@ -63,12 +63,15 @@ custom_compose: |
   version: "3"
   services:
     mysql:
-      image: mysql:5
+      image: linuxserver/mariadb
       container_name: snipe_mysql
       restart: always
       volumes:
-        - <path to mysql data>:/var/lib/mysql
+        - <path to mysql data>:/config
       environment:
+        - PUID=1000
+        - PGID=1000
+        - TZ=Europe/London
         - MYSQL_ROOT_PASSWORD=<secret password>
         - MYSQL_USER=snipe
         - MYSQL_PASSWORD=<secret user password>