Can't run adminMongo with docker-compose

I use docker-compose to run multi-container environment. But I can't integrate adminMongo into it. Here is my docker-compose.yml:

version: '2'

services:

    // other services
    adminmongo:
        build: ./adminmongo
        ports:
            - "1234:1234"
        links:
            - mongodb
        volumes:
            - "./adminmongo/app.js:/config/app.js:ro"

adminmongo/Dockerfile:

FROM mrvautin/adminmongo

EXPOSE 1234

adminmongo/app.js

module.exports = {
    "app": {
        "host": "localhost",
        "port": 1234,
        "locale": "en",
        "context": "dbApp",
        "monitoring": true
    }
}

When I run docker-compose up, the only output from this container I'm getting is:

docker_adminmongo_1 exited with code 0

If I rename app.js to app.json and change the filename in docker-compose.yml, here is what I'm getting:

adminmongo_1          | module.js:472
adminmongo_1          |     throw err;
adminmongo_1          |     ^
adminmongo_1          | 
adminmongo_1          | Error: Cannot find module '/config/app.js'
adminmongo_1          |     at Function.Module._resolveFilename (module.js:470:15)
adminmongo_1          |     at Function.Module._load (module.js:418:25)
adminmongo_1          |     at Module.runMain (module.js:605:10)
adminmongo_1          |     at run (bootstrap_node.js:420:7)
adminmongo_1          |     at startup (bootstrap_node.js:139:9)
adminmongo_1          |     at bootstrap_node.js:535:3
...
docker_adminmongo_1 exited with code 1

I'm not sure if it's the docker issue or I set up something wrong, can you please help me figuring this out?