Allow multiple instances if monitor-only

[red4dj]

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

I want to have an instance continually running with --monitor-only that notifies me of container updates. Then, I only want to trigger those updates when I run the update API command on a second instance.

Even when the --monitor-only flag is set, watchtower still does not allow multiple instances in the same scope, even though interactions between instances would be a non-issue.

Describe the solution you'd like

If a new watchtower instance is started with the --monitor-only flag, it should not clean up other instances. Also vice-versa: if a new watchtower instance is started, it should skip cleaning up any instances with the --monitor-only flag.

Describe alternatives you've considered

The only alternative I have come up with is a scheduled instance running monitoring, then to update I have to shut that down and run a second instance for a single update, then the monitoring instance has to be restarted.

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! 🙏