Improved docker image pruning #356

Job	Run time
docker_image_builds (acme-mailcow)	57s
docker_image_builds (clamd-mailcow)	1m 6s
docker_image_builds (dockerapi-mailcow)	1m 2s
docker_image_builds (dovecot-mailcow)	2m 27s
docker_image_builds (netfilter-mailcow)	1m 0s
docker_image_builds (olefy-mailcow)	1m 22s
docker_image_builds (php-fpm-mailcow)	4m 55s
docker_image_builds (postfix-mailcow)	1m 22s
docker_image_builds (rspamd-mailcow)	1m 12s
docker_image_builds (sogo-mailcow)	1m 25s
docker_image_builds (solr-mailcow)	1m 3s
docker_image_builds (unbound-mailcow)	48s
docker_image_builds (watchdog-mailcow)	50s
	19m 29s

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