Remove old docker images on the runners
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A list of images to keep is built, and all other images are removed.
The following images are kept:
- all :latest images
- all :production images
- the last three :mr-123 images
- the last three :p-123 images
- the last three :20210101.1 images
- all gitlab-runner-helper images
||string||no||Images are only removed if set to