diff --git a/Dockerfile b/Dockerfile
index 48c7a08..9868125 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -13,7 +13,9 @@ FROM composer:2 AS composer-builder
 WORKDIR /app
 COPY composer.json composer.lock ./
 # Mount auth.json as a secret so it never gets baked into the image
-RUN --mount=type=secret,id=composer_auth,dst=/composer/auth.json \
+RUN --mount=type=secret,id=composer_auth \
+    test -s /run/secrets/composer_auth || (echo "ERROR: composer_auth secret is missing or empty" && exit 1) && \
+    COMPOSER_AUTH=$(cat /run/secrets/composer_auth) \
     composer install \
     --no-dev \
     --no-interaction \