By “in your own environment”, I mean whatever you put in your CI system (in this case, whatever runs in your CircleCI workflow).
There are very few cases, in my opinion, where it is appropriate to connect to an external database to run your tests. Since you are in a Docker environment within CircleCI, it is a much more reliable (and faster) approach to spin up your own Mongo instance, insert the appropriate pieces of data for your integration tests, spin up the pieces of your app within CircleCI, and then run your integration tests (and thus nothing needs to reach outside of the CircleCI network when it is running).
To do this, you could either install Mongo in your build server, or you could spin up a Mongo instance as a secondary container (in your
docker section). How to do all this is a very broad question, and you would do well to read the manual, so you can start developing ideas about how this would work. However, if you would like some broad guidance, perhaps you could describe what different pieces you have in your system, and what the integration tests are actually doing.