There is no way to run gcp trigger via terraform. It gets invoked on certain event(code push).

Instead of creating google_dataflow_flex_template_job using terraform, use Dockerfile provided by gcp team. Build the image and push it using cloudbuild.yaml. In the same yaml, try to create dataflow jobs using gcloud dataflow flex-template build/run command.

The entire flow is, create the google_cloudbuild_trigger using terraform and providing the filename as cloudbuild.yaml along with necessary substitutions parameters. Second step is create and push docker image in cloudbuild.yaml. In the same cloudbuild.yaml, create dataflow jobs using gcloud commands.