Remove the code that generate cloud builder build files for app containers.

[ivannaranjo]

Currently the extension generates a very simple build file to build the app's container. This is not necessary as the gcloud CLI actually contains a command to build Docker images directly on the cloud builder when no complex configuration is required. The extension should call it like so:

gcloud container builds submit --tag={tag to use} {path to too of app's files}

We should only be using a build file if we intend to do more complex transformations during the build.