Use a unique header for Watchtower

[nebuk89]

Is your feature request related to a problem? Please describe.
As someone working at Docker, it would be awesome to understand how many people are using watch tower to call Docker Hub, so I can share some numbers back with the community about the impact they are having

Describe the solution you'd like
Update the user agent to be unique so we can differentiate the traffic

Describe alternatives you've considered
Not many :)

Additional context
We know there are a bunch of calls to 'containrrr' but how many more don't hit that in a given day or are going to other repos makes it hard to judge. :) Let me know if there are questions

[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! 🙏

[ghost]

To avoid important communication to get lost in a closed issues no one monitors, I'll go ahead and lock this issue. If you want to continue the discussion, please open a new issue. Thank you! 🙏🏼