Mattermost integration

[afteroot]

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

....

Describe the solution you'd like

Recieve message into Mattermost

Describe alternatives you've considered

For now funtional is close to send Slack message,
but the error in url check
Slack url https://hooks.slack.com/services/xxx/yyyyyyyyyyyyyyy"
MM url https://mattermost.com/hooks/xxxxxxxxxxxxxxxx \

So if Mattermost send message will be implemented will be appreciated

Additional context

No response

[github-actions]

Hi there! 👋🏼 As you're new to this repo, we'd like to suggest that you read our code of conduct as well as our contribution guidelines. Thanks a bunch for opening your first issue! 🙏

[piksel]

Mattermost is fully supported using shoutrrr:

https://containrrr.dev/watchtower/notifications/#shoutrrr_notifications

https://containrrr.dev/shoutrrr/v0.8/services/mattermost/

WATCHTOWER_NOTIFICATION_URL: mattermost://[username@]mattermost-host/token[/channel]

[afteroot]

@piksel Thanks a lot for your answer, will try!