feat: label released images

This will allow us to properly handle cases where the labeled image isn't actually the correct image due to invalid builds.

plugins/builder-dockerfile/builder-release

@@ -13,6 +13,7 @@ trigger-builder-dockerfile-builder-release() {
 
   # buildstep plugins don't necessarily make sense for dockerfiles. call the new breed!!!
   plugn trigger pre-release-dockerfile "$APP" "$IMAGE_TAG"
+  echo "FROM $IMAGE" | docker build --label=com.dokku.image-stage=release -t "$IMAGE" -
   plugn trigger post-release-dockerfile "$APP" "$IMAGE_TAG"
 }
 

plugins/builder-herokuish/builder-release

@@ -36,6 +36,8 @@ trigger-builder-herokuish-builder-release() {
 
     "$DOCKER_BIN" container commit "${DOCKER_COMMIT_LABEL_ARGS[@]}" "$CID" "$IMAGE" >/dev/null
   fi
+
+  echo "FROM $IMAGE" | docker build --label=com.dokku.image-stage=release -t "$IMAGE" -
   plugn trigger post-release-buildpack "$APP" "$IMAGE_TAG"
 }