Replace RUNRUNIT_CLIENT_ID and RUNRUNIT_SECRET for obtained by Runrun.it support and run the command:
export RUNRUNIT_CLIENT_ID=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
export RUNRUNIT_SECRET=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
rails s