feat: add the ability to log into a registry on a per-app basis

Closes #5324

plugins/builder-herokuish/builder-release

@@ -30,6 +30,10 @@ trigger-builder-herokuish-builder-release() {
   DOKKU_APP_USER=$(config_get "$APP" DOKKU_APP_USER || true)
   DOKKU_APP_USER=${DOKKU_APP_USER:="herokuishuser"}
 
+  local DOCKER_CONFIG
+  DOCKER_CONFIG="$(fn-registry-docker-config "$APP")"
+  [[ -n "$DOCKER_CONFIG" ]] && export DOCKER_CONFIG
+
   if ! suppress_output "$DOCKER_BIN" image build "${DOCKER_BUILD_ARGS[@]}" $DOKKU_GLOBAL_BUILD_ARGS -f "$PLUGIN_AVAILABLE_PATH/builder-herokuish/dockerfiles/builder-pre-release.Dockerfile" --build-arg APP_IMAGE="$IMAGE" --build-arg "DOKKU_APP_USER=$DOKKU_APP_USER" -t "$IMAGE" "$TMP_WORK_DIR"; then
     dokku_log_warn "Failure injecting environment variables"
     return 1