web serves the site and old-web is stopped. Look inside web and add a version file.

Find the base OS and the nginx version from inside the container:

docker exec web cat /etc/os-release
docker exec web env

Create a file nginx serves. sh -c makes the > run inside the container:

docker exec web sh -c 'echo v2.4.1 > /usr/share/nginx/html/version.txt'
docker exec web curl -s localhost/version.txt

Open a shell, create a directory, and leave:

docker exec -it web bash
mkdir /usr/share/nginx/html/maintenance
ls /usr/share/nginx/html
exit

Try the same on the stopped container and read the error: