Add custom domain to Render preview environment app

add_domain.rb

#!/usr/bin/ruby

# Script to add a custom domain to a preview environment web service. Use it via the buildCommand
#
# Resultant custom domain added to the service will be:
# 	<servicename>-pr-<number>.prs.mydomain.com
# 
# Setup:
# 
# Add a wildcard DNS entry to your domain, something like *.prs.mydomain.com pointing to 216.57.24.1
#
# Create a Render environment group and add:
# 	CUSTOM_DOMAIN_SUFFIX - set this to the DNS entry you added minus the * symbol, eg .prs.mydomain.com
#		RENDER_API_TOKEN - get one from your account settings
#
# Then, in your render.yaml
#
# envVars:
#		- fromGroup: <name of env group>

require 'uri'
require 'net/http'
require 'openssl'

# we only want to run on Render in pull requests for web services
if ENV['RENDER'] == 'true' && ENV['IS_PULL_REQUEST'] == 'true' && ENV['RENDER_SERVICE_TYPE'] == 'web' && ENV['CUSTOM_DOMAIN_SUFFIX'] && ENV['RENDER_API_TOKEN']

	CUSTOM_DOMAIN_TO_ADD = ENV['RENDER_SERVICE_NAME'] + ENV['CUSTOM_DOMAIN_SUFFIX']
	SERVICE_ID = ENV['RENDER_SERVICE_ID']

	url = URI("https://api.render.com/v1/services/#{SERVICE_ID}/custom-domains")

	http = Net::HTTP.new(url.host, url.port)
	http.use_ssl = true

	# Arbitary change

	request = Net::HTTP::Post.new(url)
	request["Accept"] = 'application/json'
	request["Content-Type"] = 'application/json'
	request["Authorization"] = "Bearer #{ENV['RENDER_API_TOKEN']}"
	request.body = "{\"name\":\"#{CUSTOM_DOMAIN_TO_ADD}\"}"

	response = http.request(request)
	puts response.read_body
	puts "Added custom domain to #{SERVICE_ID}"
	puts "Access via: https://#{CUSTOM_DOMAIN_TO_ADD}"
else
	puts "Unable to add custom domain to service, because..."
	puts "> Not Deploying to Render" if !ENV['RENDER']
	puts "> Not a pull request" if !ENV['IS_PULL_REQUEST']
	puts "> Not a web service" if !ENV['RENDER_SERVICE_TYPE'] =='web'
	puts "> Custom domain suffix not provided" if !ENV['CUSTOM_DOMAIN_SUFFIX']
	puts "> Render API token not provider" if !ENV['RENDER_API_TOKEN']
end

a RENDER_SERVICE_ID does not exist?

right now i have to grep on RENDER_INSTANCE_ID

I found that I had to take RENDER_INSTANCE_ID and do .replace(/[-][a-z0-9]+[-][a-z0-9]+$/, '') to it (to get the SERVICE_ID from the INSTANCE_ID).

Alternatively, one could get it from calling https://api-docs.render.com/reference/get-services, filtering on slug == RENDER_SERVICE_NAME.