Automatic clean-up of the outdated ExApp Docker images

[oleksandr-nc]

Describe why it is important and where it will be useful

Docker images of former versions of the ex-apps fill up the disk space.

Describe your proposed solution

1. Implement a background task for cleaning images with a configurable time interval(measurement in days, value 0 means job is disabled, default value=7).

2. Optional checkbox "clean images right after update", disabled by default.

Note: Options should be located here: http://nextcloud.local/index.php/settings/admin/app_api

Relevant API endpoint would be: https://docs.docker.com/reference/api/engine/version/v1.47/#tag/Image/operation/ImagePrune

Describe alternatives you've considered, if relevant

No response

Additional context

Additional related changes to be done in other repos in a separate PRs:

• Allow images/prune path in the haproxy config in https://github.com/nextcloud/docker-socket-proxy
• Add a high level API and allow the prune path in https://github.com/nextcloud/HaRP