It looks like the 
     219: user: "1000:50" 
statement was added a while back to fix a file permissions issue, but on a completely fresh up it keeps mysql from starting, generating the following.  Removing the line allowed everything to work properly.:

mysql_1                | Initializing database
mysql_1                | mysqld: Can't create/write to file '/var/lib/mysql/is_writable' (Errcode: 13 - Permission denied)
mysql_1                | 2017-09-14T23:55:30.989254Z 0 [Note] Basedir set to /usr/
mysql_1                | 2017-09-14T23:55:30.989559Z 0 [Warning] The syntax '--symbolic-links/-s' is deprecated and will be removed in a future release
mysql_1                | 2017-09-14T23:55:30.989602Z 0 [Warning] 'NO_ZERO_DATE', 'NO_ZERO_IN_DATE' and 'ERROR_FOR_DIVISION_BY_ZERO' sql modes should be used with strict mode. They will be merged with strict mode in a future release.
mysql_1                | 2017-09-14T23:55:30.990877Z 0 [ERROR] --initialize specified but the data directory exists and is not writable. Aborting.
mysql_1                | 2017-09-14T23:55:30.990886Z 0 [ERROR] Aborting