Three containers share the stats network. counter exited and proxy on port 8081 returns 502. Both point at localhost.

host :8081 ──► proxy ──► counter :8080 ──► redis :6379

See what failed:

docker logs counter

Recreate the counter so it finds Redis by name:

docker rm counter
docker run -d --name counter --network stats -p 8080:8080 -e REDIS_HOST=redis stackmonsters/visit-counter:1.0
curl -s localhost:8080

Now the proxy. Read its log and config:

curl -s localhost:8081
docker logs proxy
cat ~/proxy/default.conf

~/proxy is bind-mounted at /etc/nginx/conf.d. Change proxy_pass to http://counter:8080; with nano ~/proxy/default.conf, then restart nginx so it reads the new config:

docker restart proxy
curl -s localhost:8081