Support individual intervals

[gmuellerinform]

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

I want my local registry to be polled every 30 seconds, so pushes from CI/CD are quickly applied.

Describe the solution you'd like

Scan for label like com.centurylinklabs.watchtower.interval=30s and use this interval for this container only.

Describe alternatives you've considered

Start second watchtower instance

Additional context

No response