I think I figured it out (with your help). When I used your run statement, the container didn’t start. But there were a couple log entries.
root@synology:/volume1/docker# docker logs mariadb
2020-05-07 19:44:41-07:00 [Note] [Entrypoint]: Entrypoint script for MySQL Server 1:10.4.12+maria~bionic started.
2020-05-07 19:44:42-07:00 [Note] [Entrypoint]: Switching to dedicated user ‘mysql’
2020-05-07 19:44:42-07:00 [Note] [Entrypoint]: Entrypoint script for MySQL Server 1:10.4.12+maria~bionic started.
2020-05-07 19:44:43-07:00 [ERROR] [Entrypoint]: Database is uninitialized and password option is not specified
You need to specify one of MYSQL_ROOT_PASSWORD, MYSQL_ALLOW_EMPTY_PASSWORD and MYSQL_RANDOM_ROOT_PASSWORD
I had a password in the run statement, but I was using a special character. Once I removed that special char, it worked fine w/ Jason’s original run statement. 
Continuing on…
-Daren