Accessing host machine from within docker container

In my case, I am running a data visualization app (looker), which needs to whitelist the url of the main application, to allow embedding of iframes under that ip/domain. So in development, the iframes don’t resolve, because there is no way for looker to know the ip of the host machine which is running my rails app.

Basically the dependency stack is

Rails
–postgres
–rabbitmq
–looker
----mysql

Links won’t work because I, like many others from what I could gather on these boards, don’t want to run the main application (rails) in a docker container, in development. It will only make development/debugging more difficult.