Microk8s own registry on http://localhost:32000/ can get your drive full with time if you have a lot of images and/or tags; so I made a script based on solutions out in the internet about it, but it's an all in one; Just copy it to your server and run it weekly, monthly or wharever schedule fits you.

microk8s_registry_prune.sh

#!/bin/bash

# Author: Pavel Milanes <pavelmc@gmail.com>
# Goal: Prune the microk8s registry from unused images
# Date: May 2023

# force install needed tools: git, python3-pip, jq, curl...
apt-get install git python3-pip curl jq -y

# get the list of runing container images, install the soft if not there
running=/tmp/running
if [ ! -d /root/microk8s_prune/ ] ; then
    # install microk8s prune
    cd /root/
    git clone https://github.com/microk8s/microk8s.git
    cd microk8s_prune/

    # install dependencies
    pip install -r requirements.txt
fi

# get the list
env PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python \
	/usr/bin/python3 /root/microk8s_prune/microk8s_prune.py c > $running

# WARNING if the resulting file has no C: at the start of each line and at least
#  20 lines, stop! you can erase images being used!
test=$(cat $runing | grep -e "^C\: .*$" | wc -l)
if [ $test -lt 20 ] ; then
    # Fail
    echo "Warning the result of the running container is weird, not cleaning up"
    exit 1
fi

# fun starts here
registry=http://localhost:32000
repositories=$(curl ${registry}/v2/_catalog)
# cycle on the repositories
for repo in $(echo "${repositories}" | jq -r '.repositories[]'); do
    # skip is no repo
    if [ -z "$repo" ]; then continue; fi

    # get the tags for that repo
    tags=$(curl -sSL "${registry}/v2/${repo}/tags/list" | jq -r '.tags[]')

    # cycle on the tags
    for tag in $tags; do

        # check if the tag is running
        found=$(grep $tag $running)
        if [ -z "$found" ]; then
            echo "$repo/$tag is not running, can be deleted"

            # get the sha for that tag
            sha=$(curl -sSL -I \
                -H "Accept: application/vnd.docker.distribution.manifest.v2+json" \
                "${registry}/v2/${repo}/manifests/${tag}" |\
                awk '$1 == "Docker-Content-Digest:" { print $2 }' |\
                tr -d $'\r')

            # mark the tag as deteled [no real delete]
            curl -v -sSL -X DELETE "${registry}/v2/${repo}/manifests/${sha}"
        else
            echo "$tag is running, can't be deleted"
        fi
    done
done

# force registry to garbage collect images marked for deletion
pod=$(/snap/bin/microk8s kubectl get pods --namespace="container-registry" | grep Running | awk '{print $1}')
if [ "$pod" ] ; then
    # force garbage collect
    /snap/bin/microk8s kubectl exec --namespace="container-registry" ${pod} -- \
        /bin/registry garbage-collect /etc/docker/registry/config.yml
fi

If you have a low pod count and the script refuses to clean giving you an output like this:

Warning the result of the running container is weird, not cleaning up

Just adjust the count if the line 29 to match your cluster minimum pod count.