How to setup continuous deployment in Google Cloud Build?

Hello. I’m trying to setup continuous deployment in Google Cloud Build.

I have setup cloud build without testing.

steps:
  - name: "gcr.io/cloud-builders/gcloud"
    args: ["app", "deploy", "./milk/app-staging.yaml"]

Now I’m trying to setup cloud build with testing.
I searched a community-contributed builder in this repository, but I could not find one for elixir.
So I’d like to see an example of custom builder of elixir :sweat_smile: