fix: update Nginx volume path to use GITHUB_WORKSPACE variable

compose.prod.yml

@@ -52,7 +52,7 @@ services:
     image: nginx:stable-alpine
     restart: unless-stopped
     volumes:
-      - ${GITEA_WORKSPACE}/nginx/nginx.conf:/etc/nginx/conf.d/default.conf
+      - ${GITHUB_WORKSPACE}/nginx/nginx.conf:/etc/nginx/conf.d/default.conf
     depends_on:
       - frontend
       - backend