SQLSTATE[HY000][2002]No such file or directory

I've done this before with no problem, but this time I can't get past the install wizard.
Via Docker on Ubuntu Server:
Owncloud 9.1.5
Mysql 5.7.18-ldebian8

I know I'll miss something, but here's some info:
Owncloud container was created with --link MySql:mysql where MySql is the name of the mysql container.

Accordingly I specified mysql instead of localhost, but also tried local host, and and nothing works. I can connect to the mysql container via workbench on my desktop, so that appears to be working. owner:group is www-data for the apps, config and data external volumes, because I've tried everything, currently permissions are 777 for mysql and owncloud volumes until I can figure out the problem.

The volumes have been checked and re-checked. no typos there. Any help would be great!