Add example to documentation: running MySQL

[itamarst]

As followup to MongoDB tutorial in getting started section, we should have an example configuration for running MySQL. This doesn't have to a be a full tutorial, just a "here's a sample config that makes MySQL available and here's how I connected to it with postgresql client". Don't even need instructions for installation the client, at this point in docs we're presuming less hand holding.

If it's sufficiently easy a custom small image with same technique as #383, otherwise open a followup issue.

[itamarst]

We'll do this next release, and perhaps also as a blog post in the interim.

[dwgebler]

The MySQL example will now use the new environment variables application config feature, so this issue is being absorbed by #593 and will be fixed and closed by the pull request for issue 593.