mgo Docker panic

[Jean-Marcel Belmont]

db.go
```

package model

import (

 "log"

 "os"

  mgo "gopkg.in/mgo.v2"

)

// DBName is name of the database and CName is the name of document collection

const (

 DBName = "docker"

 CName  = "users"

)

var session *mgo.Session

// CreateDBSession creates mgo session

func CreateDBSession() {

 var err error

 session, err := mgo.Dial(os.Getenv("MONGO_URL"))

 defer session.Close()

 if err != nil {

   log.Fatal("Cannot Dial Mongo: ", err)

 }

 session.SetMode(mgo.Monotonic, true)

 collection := InitDB()

 CreateInitDocument(collection)

}

// GetSession will create a session if doesn't exist else will return existing session

func GetSession() *mgo.Session {

 if session == nil {

   CreateDBSession()

 }

 return session

}

// InitDB is to be used in main.go to initialize database

func InitDB() *mgo.Collection {

 c := NewContext()

 return c.DBCollection()

}

// CreateInitDocument initializes collection with 50 users

func CreateInitDocument(m *mgo.Collection) {

 models := GetModels()

 for _, model := range models {

   m.Insert(model)

 }

}

```

docker-compose.yml
```

version: '3'

services:

 db:

   image: mongo:3.4.5

   ports:

     - "27017:27017"

   volumes:

     - /usr/local/var/mongodb:/data/db

 redis:

   image: redis:3.2.9-alpine

   volumes:

     - ./redis.conf:/usr/local/etc/redis/redis.conf

   ports:

     - "6379:6379"

 frontend:

   build:

     context: .

     dockerfile: Dockerfile

   environment:

     DOCKER: "true"

   volumes:

     - .:/var/www/app/:rw

     - /var/www/app/node_modules

   ports:

     - "3000:8081"

   restart: always

 test:

   image: mhart/alpine-node:8.0.0

   working_dir: /var/www/app

   volumes:

     - .:/var/www/app

   environment:

     NODE_ENV: testing

   command:

     /bin/sh -c "./node_modules/.bin/ava test/unit/*.js"

 backend:

   build:

     context: .

     dockerfile: Dockerfile-go

   command: go run main.go

   volumes:

     - .:/var/www/app/:rw

   ports:

     - "8080:8080"

   environment:

     MONGO_URL: "db:27017/docker"

     REDIS_URL: "redis"

   restart: always

   links:

     - db

     - redis

```

Dockerfile
```

FROM mhart/alpine-node:8.0.0

LABEL maintainer "marcel...@gmail.com"

# Set Environment variables

ENV appDir /var/www/app

RUN apk add --no-cache make gcc g++ python bash bzr git subversion openssh-client ca-certificates

# Set the work directory

RUN mkdir -p ${appDir}

WORKDIR ${appDir}

COPY . ${appDir}

COPY package.json ${appDir}/package.json

# Install npm dependencies and install ava globally

RUN npm install

# Add main node execution command

CMD ["npm", "run", "dev:docker"]

```

Dockerfile-go
```

FROM mhart/alpine-node:8.0.0

LABEL maintainer "marcel...@gmail.com"

# Set Environment variables

ENV appDir /var/www/app

RUN apk add --no-cache make gcc g++ python bash bzr git subversion openssh-client ca-certificates

# Set the work directory

RUN mkdir -p ${appDir}

WORKDIR ${appDir}

COPY . ${appDir}

COPY package.json ${appDir}/package.json

# Install npm dependencies and install ava globally

RUN npm install

# Add main node execution command

CMD ["npm", "run", "dev:docker"]

```

So at the moment I have 2 different containers. I can't seem to connect to mgo in docker. I run the following command `npm run build` and `npm run dev` but the connection to mgo doesn't connect