Connect to host mongodb from docker container

So I want to connect to my mongodb running on my host machine (DO droplet, Ubuntu 16.04). It is running on the default 27017 port on localhost.

I then use mup to deploy my Meteor app on my DO droplet, which is using docker to run my Meteor app inside a container. So far so good. A standard mongodb://... connection url is used to connect the app to the mongodb. Now I have the following problem:

mongodb://...@localhost:27017... obviously does not work inside the docker container, as localhost is not the host's localhost.

I already read many stackoverflow posts on this, I already tried using:

• --network="host" - did not work as it said 0.0.0.0:80 is already in use or something like that (nginx proxy)
• --add-host="local:<MY-DROPLET-INTERNET-IP>" and connect via mongodb://...@local:27017...: also not working as I can access my mongodb only from localhost, not from the public IP

This has to be a common problem!

tl;dr - What is the proper way to expose the hosts localhost inside a docker container so I can connect to services running on the host? (including their ports, e.g. 27017).

I hope someone can help!