Bash script to clean up Google Container Registry by keeping recently images

gcr_clean_up.sh

#!/bin/bash

project=your-awesome-project-id

main() {
    for image in $(gcloud container images list --repository=gcr.io/$project); do
        if [[ $image != 'NAME' ]]; then
            # clean up dummy tags
            clean_up_image $image "NOT tags:dev* AND NOT tags:staging* AND NOT tags:production* AND NOT tags:latest" 0

            clean_up_image $image "dev" 3
            clean_up_image $image "staging" 5
            clean_up_image $image "production" 10
        fi
    done
}

# $0: image link, $1: filter, $2: number of tags want to keep
clean_up_image() {
    local count=0
    image=$1
    filter=$2
    history_limit=$3
    for digest in $(gcloud container images list-tags $image --format="get(digest)" --sort-by="~timestamp" --filter="${filter}"); do
        let count=count+1
        img_digest=$image@$digest
        if [[ $count -gt $history_limit ]]; then
            gcloud container images delete $img_digest --force-delete-tags --quiet
        fi        
    done
}

main

Can you give us some more information about this script? ex. use case of it