Using --add-host or extra_hosts with docker-compose

I am using docker-compose to run a test environment, that consists of about 5 different containers. The inter-container links and the shared volumes (volumes-from) works wonderfully. I also expose some ports up to the host machine, which works nicely.

What I am missing is a way to link some of my real servers into this environment, without hardcoding ip address. With docker run, you could use --add-host to add another line in your /etc/hosts file. Is there any way to do something similar with docker-compose?

I have great news: this will be in Compose 1.3!

I'm using it in the current RC (RC1) like this:

rng:
  build: rng
  extra_hosts:
    seed: 1.2.3.4
    tree: 4.3.2.1

https://github.com/compose-spec/compose-spec/blob/master/spec.md#extra_hosts

extra_hosts - Add hostname mappings. Uses the same values as the docker client --add-host parameter.

extra_hosts:
 - "somehost:162.242.195.82"
 - "otherhost:50.31.209.229"

An entry with the ip address and hostname will be created in /etc/hosts > inside containers for this service, e.g:

162.242.195.82  somehost
50.31.209.229   otherhost