Create a unicorn flavor of Mattermost

[Racer159]

Is your feature request related to a problem? Please describe.

As Ezra I want a unicorn package flavor of Mattermost so that I can take advantage of the better security and size posture those images afford.

Describe the solution you'd like

• Given I am logged into GHCR
• When I list tags in ghcr.io/defenseunicorns/packages/private/uds/mattermost
• Then I will see the latest version of a unicorn flavored package
• And When I include this package in my bundle
• Then it will pull/create/deploy successfully just as it would with any other flavor

Additional context

This was already accomplished for SonarQube through this issue: defenseunicorns/uds-package-sonarqube#96