docker: Use a reserved TLD for the email address

[anomiex]

Closes MONOREP-225

Proposed changes:

PHPMailer does not like an email address with a dotless domain, such as wordpress@localhost. #44983 added a plugin to rewrite this address, but chose a domain that could theoretically exist on the Internet. It would be safer to use one of the RFC 6761 reserved TLDs.

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

#44983 (comment)

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

No

Testing instructions:

• Using the Jetpack dev env, visit http://localhost/wp-admin/ and send yourself a password reset email. Check the MailPit instance to see that the email was sent.

[github-actions]

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[tbradsha]

Thanks!