Override RAILS_ENV and other environment variables on App Engine Flex

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app.rb

require "webrick"
require "yaml"

server = WEBrick::HTTPServer.new Port: ENV["PORT"] || 8080

server.mount_proc "/" do |request, response|
  response.content_type = "text/plain"
  response.body = "ENV Variables: #{ENV.to_hash.to_yaml}"
end

trap "INT" do
  server.stop
end

server.start

app.yaml

env: flex
runtime: ruby
entrypoint: ruby app.rb

env_variables:
  APP_YAML_VAR: this was set in the app.yaml
  RAILS_ENV: overridden