Using Docker to launch web app, can't connect to Postgresql DB?

Why I can NOT connect to localhost:5432?

Cat your container's /etc/hosts

$ sudo docker exec -it [container] cat /etc/hosts

For docker networks is bridge by default, the localhost inside points to container itself(Docker default bridge network). Then you don't have 5432 listening in your container:

$ sudo docker exec [container] nc -v -z localhost 5432

---

Solution 1. If you wanna hardcode the "localhost:5432" inside your config xml, the easiest way is creating your container with the option "--net=host":

$ sudo docker run --net=host -it ...

Solution 2. Change the localhost of your docker host ip inside the container

• Get your docker host ip：

  $ sudo docker inspect -f '{{ .NetworkSettings.Gateway }}' 
  192.168.5.1

• Enter your container:

  $ sudo docker exec -it [container] /bin/bash

• Edit the file /etc/hosts to point the localhost to docker host ip:

  $ sudo vim /etc/hosts
  192.168.5.1 localhost

Solution 3. Modify your db config file to use an alias instead of localhost:

connectionURL="jdbc:postgresql://DB_ALIAS/admin?stringtype=unspecified"

Then add the DB_ALIAS to the container's hosts :

$ sudo docker run --add-host DB_ALIAS:192.168.5.1 -it [image] ...

If you are using docker-compose together with postgres image, than you can reuse service name as IP inside jdbc connection (here: app-db)

  web:
    build: ./web
    ports:
      - "8080:8080"
    links:
      - app-db
    environment:
      - MYAPP_JDBC_URL=jdbc:postgresql://app-db:5432/somedb
      - MYAPP_JDBC_USER=someuser
      - MYAPP_JDBC_PASS=pass

  app-db:
    image: postgres:9.6
    environment:
      - POSTGRES_USER=someuser
      - POSTGRES_PASSWORD=pass
      - POSTGRES_DB=somedb
    expose:
      - 5432
    volumes_from:
      - app-db-data

  app-db-data:
    image: cogniteev/echo
    command: echo 'Data Container for PostgreSQL'
    volumes:
      - /opt/postgresdata/:/var/lib/postgresql/data

I was getting the same error but this simple solution works perfect for me.
sudo docker run -d --net="host" -it <IMAGE> Now I can run my app https://x.x.x.x:pppp/../.. and everything works fine. I hope this helps