remove_old_docker_images.sh

#!/bin/bash
set -e

sort_func () {
    tr ' ' '\n' | sort -u | (grep -v '^$' || true)
}
cutoff=$(date -Is --date='10 days ago')

# Get list of images used recently
# NOTE: events store image names, not IDs.
# NOTE: events may reference images which no longer exist.
used_image_names=$(docker events --since "$cutoff" --until="$(date -Is)" --format '{{.From}}' | sort_func)
echo Recently used image names: "$(wc -w <<<"$used_image_names")"
used_image_ids=$( (xargs -r -n1 docker image inspect --format='{{.ID}}' <<<"$used_image_names" 2>/dev/null || true) | sort_func)
echo Recently used image ids: "$(wc -w <<<"$used_image_ids")"

# Get list of images with old CreatedAt dates
old_image_ids=''
for image_id in $(docker image ls -q); do
    if [ "$(docker inspect --format='{{.Created}}' --type=image "$image_id")" \< "$cutoff" ]; then
        old_image_ids+=" $(docker image inspect "$image_id" --format='{{.ID}}')"
    fi  
done
old_image_ids=$(sort_func <<<"$old_image_ids")
echo Old image IDs: "$(wc -w <<<"$old_image_ids")"

# Remove old images which have not been recently used
to_remove=$(comm -23 <( echo "$old_image_ids" ) <( echo "$used_image_ids" ) )
echo Image IDs to remove: "$(wc -w <<<"$to_remove")"
for image_id in $to_remove; do
    # NOTE: `docker rmi image_id` doesn't work if the image has multiple tags.
    # NOTE: `docker rmi --force` is not safe if there is a running container.
    for image_name in $(docker inspect "$image_id" --format='{{join .RepoTags " "}}') "$image_id"; do
        docker image rm "$image_name" || true
    done
done