Add mock email server to development environment

[williamjallen]

Email testing is currently difficult in the CDash development environment, with developers having to set up their own local email servers for testing purposes. This PR adds a mailpit container to the Docker development environment so developers can inspect the emails sent by a local CDash server at http://localhost:8025.

[josephsnyder]

@williamjallen, this starts up nicely but I can't see any emails come into the window using docker/mysql.

Can we get the documentation updated with any further steps that need to be taken to use this?

[williamjallen]

@josephsnyder there shouldn't be any more steps needed long-term. For now, it looks like the CDash function used to send mail currently returns when in debug mode. You could either try running CDash in production mode, or add a custom email command somewhere to try things out. I used something along the lines of this for example:

Mail::raw('test', function ($message) {
    $message->to('[email protected]')->subject('test');
});

I have another PR queued up behind this one, which will remove the function I linked above and allow all emails sent as part of the suite to show up in mailpit.

[josephsnyder]

Awesome, thanks for the APP_DEBUG pointer. Much better!