Do not remove images that did not exist in the last run

docker-gc

@@ -231,11 +231,15 @@ xargs -n 1 $DOCKER inspect -f '{{.Image}}' 2>/dev/null |
 sort | uniq > images.used
 
 # List images to reap; images that existed last run and are not in use.
+echo -n "" > images.last-run
+if [ -f images.all ]; then
+    cat images.all > images.last-run
+fi
 $DOCKER images -q --no-trunc | sort | uniq > images.all
 
 # Find images that are created at least GRACE_PERIOD_SECONDS ago
 echo -n "" > images.reap.tmp
-cat images.all | while read line
+comm -23 images.last-run images.all | while read line
 do
     CREATED=$(${DOCKER} inspect -f "{{.Created}}" ${line})
     ELAPSED=$(elapsed_time $CREATED)