Images retention options

[Tassleh0ff]

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

Recently one of the images I use in mu containers had an update which blocked the service because of a bug, but I can't start the container with the previous image because it was deleted by the WATCHTOWER_CLEANUP=true option.

Describe the solution you'd like

I would be useful if there's an option to set the number of images to retain on the system instead of delete all the images not used by some container, something like WATCHTOWER_CLEANUP_RETENTION with a number parameter.
For example:
WATCHTOWER_CLEANUP_RETENTION=N to retain the latest N images

Describe alternatives you've considered

As an alternative I have to disable the unused images deletion and make an external script to sort them out by date and manage the retention with it.

Additional context

No response

[github-actions]

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