Support polling time splay / randomization

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

When many separate machines are using watchtower to poll for the same image updates, it's possible to run into a thundering herd type of problem, where many machines are updating their services all at the same time. This can be especially true for environments where the same watchtower configuration has been deployed simultaneously using a mass-configuration management system (Ansible, Chef, SaltStack, Puppet, etc).

Describe the solution you'd like

I believe the problem could be alleviated by introducing a new configuration option to splay the timing of watchtower's polling interval and schedule option values. The argument to the splay option could be expressed as a number of seconds to insert as a random delay to the interval or schedule timing. This random delay would be re-calculated each time the interval or schedule is triggered.

Examples:

Poll every 5 to 6 minutes: watchtower --interval 300 --splay 60

Schedule once every day between 12:00 and 12:10 watchtower --schedule "0 12 * * *" --splay 600

Describe alternatives you've considered

An alternative could be to add a feature to watchtower that uses a network service like etcd, where watchtower would wait for a "lock" to be opened before it starts performs its poll. This may be a more robust approach for some cases but would likely be more complex.

Additional context

No response

containrrr / watchtower