How to connect to external Mongo (MLab) from a Node app in Docker - mongodb

Question

I have a database setup on Mlab and I have aconnection string that does work remotely. If I run my node app on docker using my local mongo, everything works as expected, however if I change the configuration and connect to my Mlab mongo, I get the following error:
UnhandledPromiseRejectionWarning: MongoNetworkError: failed to connect to server [localhost:27017] on first connect [MongoNetworkError: connect ECONNREFUSED 127.0.0.1:27017]
at Pool.<anonymous> (/usr/src/app/node_modules/mongodb-core/lib/topologies/server.js:564:11)
at emitOne (events.js:116:13)
at Pool.emit (events.js:211:7)
at Connection.<anonymous> (/usr/src/app/node_modules/mongodb-core/lib/connection/pool.js:317:12)
at Object.onceWrapper (events.js:317:30)
at emitTwo (events.js:126:13)
at Connection.emit (events.js:214:7)
at Socket.<anonymous> (/usr/src/app/node_modules/mongodb-core/lib/connection/connection.js:246:50)
at Object.onceWrapper (events.js:315:30)
at emitOne (events.js:116:13)
at Socket.emit (events.js:211:7)
at emitErrorNT (internal/streams/destroy.js:66:8)
at _combinedTickCallback (internal/process/next_tick.js:139:11)
at process._tickDomainCallback (internal/process/next_tick.js:219:9)
(node:24) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 6)
Apart from handling the error on Node side (which I'm not catching, but that's another problem), how can I connect to an external mongo server form my Docker container?
I've seen this and follow the steps described there but it didn't help me.
Any suggestions?
EDIT
This is a Node app. In my Dockerfile I have the following setting:
ENV DB_HOST=mongodb+srv://<user>:<password>#<mlab>/<db_name>?retryWrites=true
And in my Node app where I have setup the database I have this:
mongoose.connect(process.env.DB_HOST, {useNewUrlParser: true, useFindAndModify: false, useCreateIndex: true});
When I print the above env variable at run time, I get the correct string for the db (running locally everything works fine).
I don't understand why the error shows localhost. This is my first time using Docker. Do I need any additional configuration so my container can talk to an external service?