add(book): Describes custom Testnets in the Zebra book #1269

	# Workflow patches for skipping Google Cloud CD deployments on PRs from external repositories.
	name: Deploy Nodes to GCP

	# Run on PRs from external repositories, let them pass, and then Mergify will check them.
	# GitHub doesn't support filtering workflows by source branch names, so we have to do it for each
	# job.
	on:
	pull_request:

	# IMPORTANT
	#
	# The job names in `cd-deploy-nodes-gcp.yml`, `cd-deploy-nodes-gcp.patch.yml` and
	# `cd-deploy-nodes-gcp.patch-external.yml` must be kept in sync.
	jobs:
	# We don't patch the testnet job, because testnet isn't required to merge (it's too unstable)
	build:
	name: Build CD Docker / Build images
	# Only run on PRs from external repositories, skipping ZF branches and tags.
	if: ${{ startsWith(github.event_name, 'pull') && github.event.pull_request.head.repo.fork }}
	runs-on: ubuntu-latest
	steps:
	- run: 'echo "Skipping job on fork"'

	test-configuration-file:
	name: Test CD default Docker config file / Test default-conf in Docker
	# This dependency allows all these jobs to depend on a single condition, making it easier to
	# change.
	needs: build
	runs-on: ubuntu-latest
	steps:
	- run: 'echo "Skipping job on fork"'

	test-configuration-file-testnet:
	name: Test CD testnet Docker config file / Test default-conf in Docker
	needs: build
	runs-on: ubuntu-latest
	steps:
	- run: 'echo "Skipping job on fork"'

	test-zebra-conf-path:
	name: Test CD custom Docker config file / Test custom-conf in Docker
	needs: build
	runs-on: ubuntu-latest
	steps:
	- run: 'echo "Skipping job on fork"'

Provide feedback