The payload is expected to be JSON with the following fields:
repo - Full name of the repository to clean, in the format
gcr.io/project/repo. This field is required.
keep - If an integer is provided, it will always keep that minimum number
of images. Note that it will not consider images inside the grace duration.
allow_tagged - If set to true, will check all images including tagged.
If unspecified, the default will only delete untagged images.
tag_filter - Used for tags regexp definition to define pattern to clean,
requires allow_tagged must be true. For example: use -tag-filter "^dev.+$"
to limit cleaning only on the tags with begining with is dev. The default
is no filtering. The regular expression is parsed according to the Go regexp package syntax.