Relates to https://github.com/coder/dogfood/pull/189
Closes coder/internal#1021

• Adds new script scripts/should_deploy.sh which implements the algorithm in the linked issue
• Changes the ci.yaml workflow to run on release branches
• Moves the deployment steps out of ci.yaml into a new workflow deploy.yaml for concurrency limiting purposes
• Changes the behavior of image tag pushing slightly:
  • Versioned tags will no longer have a main- prefix
  • main branch will still push the main and latest tags
  • release/x.y branches will now push release-x.y tags
• The deploy job will exit early if should_deploy.sh returns false
• The deploy job will now retag whatever image it's about to deploy as dogfood

TODO:

• Test the workflow out a bit, will require a lot of commenting out