Docker container cannot reach other container via reverse-proxy

I’m using docker-compose.yml for a couple services behind an nginx proxy.

I’m having the issue where my docker containers cannot reach each other using their publicly available proxy URLs. They can only reach each other using their docker service names.

The easiest way to demonstrate is as follows. I can connect to my Postgres instance from anyhere with the following:

psql -Atx postgres://username:[email protected]/database

But if I enter one of my running containers:

docker compose exec backend sh

Then try to connect using the same connection string, it just hangs… No logs even on the proxy side. But, I can make it work by using the docker service name instead:

psql -Atx postgres://username:pw@postgres:5432/database

I should be able to use postgres.mysite.com from anywhere, outside or inside the container. But for some reason I can only use it outside the container. Here’s the nginx config section for this:

   upstream docker-postgres {
      server postgres:5432;
   }

   server {
      listen 443 ssl;
      server_name postgres.mysite.com;

      location / {
         proxy_pass http://docker-postgres;
      }
   }

But this is not related to postgres. This appears to be happening regardless of the service. Whenever container A (the app code) tries to connect to container B (the service) by using the publicly available URL of the proxy running in container C.

For example, for my MinIO service, uploads hang when uploading to https://assets.mysite.com but not when uploading to http://minio:9001.

Partial docker-compose.yml:

proxy:
    image: nginx:alpine    
    ports:
      - '80:80'
      - '443:443'    
    networks:
      - myNetwork
backend:  
    ports:
      - '3000:3000'
    volumes:
      - ./:/app
      - /app/node_modules    
    networks:
      - myNetwork
postgres:
    image: postgres:10.4
    ports:
      - "5432:5432"   
    networks:
      - myNetwork   
minio:
    image: minio/minio
    command: server --console-address ":9090" --address ":9001" ./minio_data 
    ports:
      - '9090:9090'
      - '9001:9001'
    networks:
      - myNetwork

networks:
  myNetwork:
    external: true
 

Additional Information

I set up a /test endpoint in the proxy under the assets.mysite.com server_name.

location /test { return 200 'success!!'; }

I can hit this endpoint from the server (outside a docker container):

curl https://assets.mysite.com/test —-> Success

However, I can’t hit it from inside any container:

docker compose exec backend curl https://assets.mysite.com/test —> Hangs!

Despite that, I can ping assets.mysite.com successfully, even from the containers:

docker compose exec backend ping assets.mysite.com