Simple HTTP server that responds to GET and POST requests on every route with your (custom) maintenance message and status 503.
Write your message in a file (e.q. my-message.txt):
Server is undergoing maintenance.
Then run:
docker run -d -p 80:80 -v $PWD/my-message.txt:/message.txt judge0/maintenance:0.1.0
Visit http://<Server IP>.