Docker mounted volume removes node_modules

Dockerfile

# Set the base image to stable version of node
FROM node:0.12.2

# Install nodemon
RUN npm install -g nodemon

# Provides cached layer for node_modules
COPY package.json /tmp/package.json
RUN cd /tmp && npm install --production
RUN mkdir -p /app && cp -a /tmp/node_modules /app/

RUN mkdir /app/.tmp

# Define working directory
WORKDIR /app
COPY . /app

ENV DOCKER=true

# Expose port
EXPOSE  3002

# Run app using nodemon
CMD ["nodemon", "/app/index.js"]

index.js

/**
 *  This file is used for creating a symlink between the actual ./node_modules folder
 *  and the ./src/node_modules "folder".
 *
 *  This is due to being able to both run the app smoothly in a local and containerized environment.
 */

var fs = require('fs');

// Paths
var NODE_MODULES = '../node_modules'
var SRC_NODE_MODULES = './src/node_modules';

// Check if the symlink is already existing..
fs.exists(SRC_NODE_MODULES, function(exists) {

  // ?: Is it NOT existing?
  if (!exists) {
    // -> Nope, then create it and launch the server
    createSymLink(launchServer);
    return;
  }
  // E -> The symlink exists!
  console.log('! SYMLINK exists, recreate it..')
  // Remove the existing link (why? Because we do not know if it was created locally or in a container)
  fs.unlink(SRC_NODE_MODULES, function(err) {

    if (err) return console.log(err);

    // Double check that the link was removed
    fs.exists(SRC_NODE_MODULES, function(exists) {

      console.log('! SYMLINK was ' + (exists ? 'NOT DELETED!' : 'deleted successfully..'));

      // Was it removed?
      if (exists) {
        // -> NO! Then abort mission!
        return console.log('! Aborting server launch..')
      }
      // E -> It was removed successfully. Re-create it and launch the server
      createSymLink(launchServer);
    });
  });
});

var createSymLink = function(callback) {

  fs.symlink(NODE_MODULES, SRC_NODE_MODULES, 'dir', function(err) {

    if (err) return console.log(err);

    console.log('! SYMLINK created!')

    callback();
  });

}

var launchServer = function() {
    console.log('! Launching server..')
    require('./src/app.js');
}