AURCache Stack: fix missing path for build cache volume

Docker/Docker-Compose/AURCache/docker-compose.yml

@@ -6,7 +6,7 @@ services:
       - "${WEBUI_PORT:-8081}:8081" # Frontend
     volumes:
       - ${REPO_PATH:-./repo}:/app/repo
-      - ${BUILD_CACHE_PATH:}
+      - ${BUILD_CACHE_PATH:-./build_cache}:/app/builds
     privileged: true
     environment:
       - DB_TYPE=POSTGRESQL