The "internal" Docker build type is a simple way to build your Docker image using the resources from your own cluster and nothing else.
The only requirement is that your Docker image has to be buildable using the Dockerfile.
You can select the "internal" builder for two reasons:
- You don't want or need to use a 3rd party Continuous Integration (CI) system like Travis CI (free or paid)
- You want to build locally on your own cluster for speed, cost or reliability.