bk: skip removing images from hook

Signed-off-by: Ivo Jimenez <ivo.jimenez@gmail.com>

Author: ivotron

.buildkite/hooks/post-checkout

@@ -14,4 +14,6 @@ for id in $(docker ps --quiet); do
   docker kill $id
 done
 
-docker system prune --all --volumes --force
+docker container prune --force
+docker volume prune --force
+docker network prune --force