Merge pull request #781 from expa/master

use DOKKU_IMAGE (i.e. progrium/buildstep)

dokku

@@ -42,7 +42,7 @@ case "$1" in
 
   build)
     APP="$2"; IMAGE="dokku/$APP"; CACHE_DIR="$DOKKU_ROOT/$APP/cache"
-    id=$(cat | docker run -i -a stdin $DOKKU_DISTRO /bin/bash -c "mkdir -p /app && tar -xC /app")
+    id=$(cat | docker run -i -a stdin $DOKKU_IMAGE /bin/bash -c "mkdir -p /app && tar -xC /app")
     test $(docker wait $id) -eq 0
     docker commit $id $IMAGE > /dev/null
     [[ -d $CACHE_DIR ]] || mkdir $CACHE_DIR