Solve mariabackup-fails-on-docker-toolbox problem

[rfay]

The Problem/Issue/Bug:

Our latest db container is built with mariadb 10.2.19. And it doesn't work at all with docker toolbox on ddev restore-snapshot (or perhaps snapshot creation?)

See logs : https://gist.github.com/rfay/743377f93424b0f19df09cbfdae64e32

How this PR Solves The Problem:

• The latest push of our container got a 4-patch-upgrade from mariadb 10.2.15 (on v1.5.1) to 10.2.19. It does seem to me that the behavior of mariabackup changed, and that we have to add --skip-innodb-use-native-aio ; native aio is apparently the default now, although I don't see that in the changelog.
• Pin the version of mariadb installed in the container so hopefully this kind of thing won't happen to us in the future.

Manual Testing Instructions:

Use ddev snapshot and ddev restore-snapshot on Docker Toolbox.

Automated Testing Overview:

Related Issue Link(s):

Release/Deployment notes: