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

I received the following error when trying to write session data using Tomcat's PersistentManager to a Postgres DB running on my local machine:

SEVERE: A SQL exception occurred org.postgresql.util.PSQLException: Connection to localhost:5432 refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections.

The application itself runs in a docker container. For completeness sake, my current context.xml file is:

<?xml version='1.0' encoding='utf-8'?>
<Context>
<Manager className="org.apache.catalina.session.PersistentManager"
    distributable="true"  processExpiresFrequency="6" maxIdleBackup="0" debug="99" >
    <Store className="org.apache.catalina.session.JDBCStore"
        driverName="org.postgresql.Driver"
        connectionURL="jdbc:postgresql://localhost/admin?stringtype=unspecified"
        connectionName="admin" connectionPassword="admin"
        sessionAppCol="app_name" sessionDataCol="session_data" sessionIdCol="session_id"
        sessionLastAccessedCol="last_access" sessionMaxInactiveCol="max_inactive"
        sessionTable="tomcat_sessions_tb" sessionValidCol="valid_session" />
</Manager>
</Context>

Pursuant to the suggestions here: Postgresql : Connection refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections

I confirmed via a netstat -aln | grep LISTEN that Postgresql is running and listening on the correct ports:

tcp4       0      0  127.0.0.1.5432         *.*                    LISTEN     
tcp6       0      0  ::1.5432                                      *.*                                           LISTEN

and that my postgresql.conf (located in usr/local/var/postgres) has listen_addresses = localhost and port = 5432, which mirrors the host and port of my running server in Pgadmin3.

I suspect that the problem is that Docker runs in a VM, and thus the local information I have obtained may not be the whole story. Reading up on the available information online, it seems that I may require some sort of bridged networking.
However, I admit I am a novice in this area, and I'm unsure of how to set it up.

Solution

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] ...