switch from MySQL docker images to MariaDB

This seems to address some rare occasions where MySQL conversions causes issues as in https://github.com/pixelfed/pixelfed/issues/2989

docker-compose.yml

@@ -55,7 +55,7 @@ services:
 
 ## DB and Cache
   db:
-    image: mysql:8.0
+    image: mariadb:jammy
     restart: unless-stopped
     networks:
       - internal