Merge pull request #2236 from dokku/2235_mh-hide-img-check-err-output

no need to show output from is_image_herokuish_based()

plugins/common/functions

@@ -196,7 +196,7 @@ is_image_herokuish_based() {
   declare desc="returns true if app image is based on herokuish"
   # circleci can't support --rm as they run lxc in lxc
   [[ ! -f "/home/ubuntu/.circlerc" ]] && local DOCKER_ARGS="--rm"
-  docker run "$DOKKU_GLOBAL_RUN_ARGS" --entrypoint="/bin/sh" $DOCKER_ARGS "$@" -c "test -f /exec"
+  docker run "$DOKKU_GLOBAL_RUN_ARGS" --entrypoint="/bin/sh" $DOCKER_ARGS "$@" -c "test -f /exec" &> /dev/null
 }
 
 is_number() {