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

Closes #5324

plugins/builder-railpack/builder-build

@@ -125,7 +125,10 @@ trigger-builder-railpack-builder-build() {
 
   eval "$(config_export app "$APP" --merged)"
 
-  # shellcheck disable=SC2086
+  local DOCKER_CONFIG
+  DOCKER_CONFIG="$(fn-registry-docker-config "$APP")"
+  [[ -n "$DOCKER_CONFIG" ]] && export DOCKER_CONFIG
+
   if ! railpack build "${RAILPACK_ARGS[@]}" --name "$IMAGE-build" "$SOURCECODE_WORK_DIR"; then
     dokku_log_warn "Failure building image"
     return 1