Sharing a docker mount

I’m playing with Percona’s xtrabackup which tries to make a logical level backup of stuff in /var/lib/mysql.

https://s.natalian.org/2019-05-02/docker_dir.txt

But as you can see I run into file permissions errors (I think), like:
xtrabackup: File ‘/var/lib/mysql/binlog.000003’ not found (OS errno 2 - No such file or directory)

Does this look ok?

$ mkdir /tmp/datadir                                                                                                      
$ chmod 777 /tmp/datadir/                                                                                                 
$ chmod g+s /tmp/datadir/     

And then launching:

version: '2'
services:
    db:
      image: percona:ps-8
      ports:
        - 3306:3306
      environment:
            MYSQL_ROOT_PASSWORD: ${MYSQL_ROOT_PASSWORD}
      volumes:
            - /tmp/datadir:/var/lib/mysql
            - ./sql/:/docker-entrypoint-initdb.d/
      networks:
            - network


networks:
    network:
        driver: bridge

I’m an Arch user btw.