The easiest tool to compose a Docker container image is still using the docker build command with a Dockerfile. It is very common to have a cloudbuild.yaml file with a single step that uses the Dockerfile in the root of your source, like so:

steps:
- name: 'gcr.io/cloud-builders/docker'
  args: ['build', '-t', 'gcr.io/${PROJECT_ID}/my-image', '.']
images: ['gcr.io/${PROJECT_ID}/my-image']

So, for constructing a final container image, using docker build and a Dockerfile is the recommended approach. It will let you easily install dependency packages, for instance.

You can use your cloudbuild.yaml to perform other operations, before or after your docker build step. For instance, if you wanted to create a binary without packaging your SDK into the final image, it is easy to do that with cloudbuild.yaml. Or, if you wanted to pull in some extra assets from Cloud Storage using gsutil.

Container builder will also let you build and push as many images as you like, all tied to the same commit of your git repo (and they can include the commit sha in the image tag too) by having many steps that run docker build. You could even run these builds in parallel.

See https://cloud.google.com/container-builder/docs/api/build-steps for how to add steps for arbitrary things.