Good question, I actually hit this problem. In one of my projects, I build 3 microservices out of a single GitHub repository. So I had 3 different Dockerfiles. What didn't work out for me was that Cloud Container Builder does not let you specify the Dockerfile name (but it lets you specify the Dockerfile directory, so if your dockerfiles are in separate dirs, you're good; however it's not very helpful if you need to host all your dockerfiles in the repo root.). If you think providing a custom Dockerfile name would be a useful feature, please let the folks know here. I am sure they would be interested in your feedback.

That said, I was easily able to unblock myself because Cloud Container Builder does not actually rely on Dockerfile. You can in fact specify build steps arbitrarily, like Travis CI (or other CI services) by writing a json/yaml file. See https://cloud.google.com/container-builder/docs/api/build-re... and https://cloud.google.com/container-builder/docs/api/build-st.... In that file, you can specify what you want to build and how you want to build it. It is quite flexible.

We've tried to satisfy the 90% case with the UI and "easy" options, especially those that transfer well from other push-to-build products.

To keep the UI simple and understandable for everyone, we decided to drop some options and require that you use a config for the more interesting cases.

There are lots of examples of config files (often named `cloudbuild.yaml`) in our github repo: https://github.com/googlecloudplatform/cloud-builders