Docker: Change from MailDev to Mailpit

[tbradsha]

Closes MONOREP-86

This changes our use from MailDev to Mailpit.

Both are comparable, and few devs will use most of their capabilities beyond receiving email. In the end the biggest factor for me was size: Mailpit is almost 1/4 the compressed size, which makes for a quicker pull, particularly on slow connections.

Bonus:

• Two ENV vars in the Dockerfile were updated to use the modern key=value format instead of key value, which removes two deprecation notices.
• A new PORT_PHPMY var was added to allow people to configure the phpMyAdmin port.
• PORT_MAILDEV was updated to PORT_INBOX to make it generic. It's unlikely many (if any) use this, so rather than implementing a fallback that handles both, I just directly renamed it.

Proposed changes:

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

1. Check out the PR.
2. jetpack docker build-image
3. Verify http://localhost:1080/ loads
4. Do a password reset on some WP user and verify the email comes through.

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[manzoorwanijk]

Works well for me

[tbradsha]

Thanks for the review. I'll merge this once I've got a post ready.