EADDRNOTAVAIL when Dockerizing Node.js app - node.js

Question

With the following app, I am able to start it manually via npm install / node app.js. The issue is with trying to run the app via a Docker container.
Apart from what the rest of the app is (which doesn't matter because running the Docker container doesn't even get that far), the Dockerfile pulls the code from GitHub, switches to that directory, then runs app.js (which pulls the host and port from a config.json file). Attempting to run the container with the IP address of the server (what it should be listening on) results in the following error:
uncaughtException: Error: listen EADDRNOTAVAIL
Error: listen EADDRNOTAVAIL
at errnoException (net.js:905:11)
at Server._listen2 (net.js:1024:19)
at listen (net.js:1065:10)
at net.js:1147:9
at dns.js:72:18
at process._tickCallback (node.js:442:13)
at Function.Module.runMain (module.js:499:11)
at startup (node.js:119:16)
at node.js:929:3
Changing the port does nothing to resolve the issue. I've included the relevant files below (with certain sections replaced with {pseudocode}). Any help would be immensely appreciated, because I'm absolutely hopeless when it comes to Docker.
Dockerfile:
FROM centos:centos6
RUN rpm -Uvh http://download.fedoraproject.org/pub/epel/6/i386/epel-release-6-8.noarch.rpm
RUN yum install -y npm git
RUN git clone {repo.git}
COPY . /src
RUN npm install
CMD ["node", "app.js"]
config.json:
{
"app_host": "{IP of server}",
"app_port": "20000"
}
app.js:
var server = app.listen(config_json.app_port, config_json.app_host, function () {
var host = server.address().address;
var port = server.address().port;
console.log('\n listening at http://%s:%s', host, port);
});
module.exports = app;

Answer 1

Turns out robertklep in the comments to my question was correct: the issue was trying to explicitly pass the IP of the server into app.js. That's the way the app was configured to work just by itself with Node, but that can't be done with Docker.
The only code change needed was removing config_json.app_host from app.listen in app.js. Then, running the container by binding the exposed port to any available port on the server causes it to work.