Fix Windows mount of .ddev/mysql configuration (Support for Docker 18.06)

[rfay]

The Problem/Issue/Bug:

With the move to Docker 18.06 something had to break, and our container test for ddev-dbserver broke on windows. It turns out that mysqld won't (at least now) read config that is world-writable, as we've run into other places on ~/.my.cnf; I don't know why this would show up with a docker version change.

How this PR Solves The Problem:

• Change the test to mount ro
• Change so ddev generates a docker-compose.yaml that has this mount set to ro so our users don't discover this.

Manual Testing Instructions:

• If the tests success on the windows container that's good
• For extra credit try changing config in .ddev/mysql/<something.cnf> ON WINDOWS and see that your change takes effect.

[rfay]

This is on the right track, but not done yet. What's going on here is if you manually make the my.cnf read-only (in the container) then it passes tests.

There are two problems I think:

• if people using windows provide a writable whatever.cnf override file, the server won't respect it.
• Making the whole mount ro doesn't allow the container to create the mysql directory, which may not exist, and mysqld wants to importdir a whole directory.

[rfay]

Hoping this one may have all it needs. Starting nightly on it as well.

[rfay]

Yay 8 successes!

[andrewfrench]

Tests are back to passing, great! I'm not currently able to test manually on Windows, so I'll have to lean on the testbots.