I have following config:

version: "3.8"
configs:
  mycustom:
   file: ./mycustom.cnf

services:
  db:
    image: mariadb:10.2
    configs:
      - source: mycustom
        target: /etc/mysql/conf.d/custom.cnf-test
        uid: '10000'
        gid: '10000'
        mode: 0440

the file is mounted, but the permission mask is totally ignored (I tried removing leading 0 from octal, other uid/gid, nothing matters):

root@03027f798793:/# ls -al /etc/mysql/conf.d/
total 20
drwxr-xr-x 1 root root 4096 Apr 25 07:44 .
drwxr-xr-x 1 root root 4096 Aug  2  2022 ..
-rwxrwxrwx 1 root root  222 Apr 24 12:48 custom.cnf-test
-rw-r--r-- 1 root root  527 May 18  2022 mariadb.cnf
-rw-r--r-- 1 root root   36 May 18  2022 mysqld_safe_syslog.cnf